But 38-year-old Rennie is not talking about the Dunfermline and West Fife by-election which saw him wipe out a 12,000 Labour majority and win the seat for the Liberal Democrats. He's talking about the coal race - a popular but little-emulated event held during the Kelty Gala when 15 men race the length of the High Street carrying a hundredweight sack of coal on their backs.
"It's a huge bag," grins Rennie "If you keep standing upright, the muscles in your back can take it. But if you start to stoop, you're finished." The comparisons with politics are almost irresistible, except that Rennie came second in the coal race - coached as he was by its five-times winner - and a very clear first in the race to Westminster. Though he and his campaign team were convinced he could - and obviously should - win, one week on from the streamers and popping Champagne corks, the euphoria has barely subsided. In the soon-to-be-deserted constituency HQ in Inverkeithing (they will now move to bigger premises in Dunfermline) every helper is still grinning from ear to ear. Every mundane task has suddenly been transformed from tedious to delightful. They trill and laugh on the phone, and clap each other on the back for no reason at all. The damp greyness of a bleak winter afternoon is "brisk" here. Even the digestive biscuits look strangely pleased with themselves. Not since David Steel rather unwisely advised party conference delegates to return to their constituencies "and prepare for government" has the glow of Lib Dem rosette gold shone as warmly.
And without any obvious assistance from the surroundings. Inverkeithing in February is stern as a John Knox sermon, though quite a lot colder. There are few shoppers in the impressive High Street, probably because there are relatively few shops. The Fair Trade Coffee House has gone out of business, as has "Maureen's" and a note on the door of The Happy Palace says "closed", which seems poignantly appropriate. Scotland's happy palace has long suffered from short opening hours. But the town's two chip shops are doing a roaring trade - each crammed with schoolchildren whose favoured lunch-time order turns out to be half a deep-fried pizza and chips for 2.50. Jamie Oliver would be horrified. Yet 60 years ago this was a prosperous town, with a backbone of mining and shipyards. Now the estate agent's window displays a more modern divide, with the bulk of properties for sale at prices between 45,000 and 100,000, and just two priced at over 200,000, both noting their easy commuting distance from Edinburgh. Hence the threatened 4 Forth Road Bridge toll is a major local issue, and top of Rennie's brand new "urgent" list.
"I was driving back from the airport earlier in the week - coming back over the bridge for the first time in daylight since I'd been elected," says Rennie, sipping his mug of tea, "and I just thought, wow, that's my constituency! It was an amazing view, and I thought - how fantastic."
He pauses, a beatific smile still playing at the corners of his mouth, before he adds: "Of course, it's quite a huge pressure in some ways. This is a very big responsibility. There are some serious problems that need decent help. The voters want someone to sort these out. They deserve that ..." but Rennie can't keep hold of the stern, invigorating tone any longer. The boyish grin creeps back. "Overall, the feeling is just fantastic," he sighs.
Though "steady" may well have been the by-word of his political career to date, there is no denying that the last four weeks have been an astonishing sprint. A combination of tireless door-stepping, rallies, speeches and very little sleep. He had been selected in December as Lib Dem candidate for Dunfermline West for the Scottish Parliamentary election. "Then Rachel Squire died in January and they asked if I'd like to stand for the by-election. It gave us just over a month to campaign. Labour likes to keep things as short as possible so nobody else gets a chance. Of course, I thought we had more than just a chance ..."
Rennie grew up in Strathmiglo where his father Sandy ran the general store. His mother, Peta, was daughter of the manse, and remained heavily involved in community activities while she brought up Willie and his three elder sisters. "A very determined woman, my mother," he says. "She was chairman of the community council, and very involved in her father's church. It was my other grandfather who started the shop in 1964. At that time they also ran vans to sell stuff round the farms, and they also had a shop in Cupar, but eventually it was just the Strathmiglo shop which my Dad took over. He worked dreadful hours. From 6am to midnight in the early years."
The family lived over the shop. "It was very warm because the heat from all the fridges and chill cabinets came up through the floor. But there was a pub right opposite, so it could a bit noisy at night." Though Rennie admits that, as the youngest and the only boy, he was "spoiled rotten", he was a courageous, outgoing child. "My mother tells me I liked to stand on the wall outside the house when I was about four, saying hello to anyone who passed by. It was a high wall, so she was always telling me to come down in case I fell." But climbing walls was just a start. Haystacks came next, and soon after that, a lifelong devotion to hill-climbing and running.
"I've run up Ben Nevis with my cousin," he tells me.
Actually run? I query.
"Oh yes. Well, there are bits where you just walk - or climb - because you're stepping upwards; otherwise you can jog quite easily. It takes eight hours if you're walking, but we did it at two hours 25; so we were going at a fair pace."
The sort of chap who can run up Ben Nevis probably would take a Fife by-election in his stride. I ask - as such activities are a mystery to me - if he mulls over everyday problems while he's running, or if he clears his mind. "Some runners say they disengage the brain, but I like to watch out for wildlife and enjoy what I'm seeing."
Biology was the only subject Rennie really enjoyed at Bell Baxter High School in Cupar. "I can't say I enjoyed school much at all, but my biology teacher, Mr Shilson, was an amazing man. He really knew how to inspire, and make you want to find out more. So I became fascinated by wildlife." After school he applied to Paisley College to study ecology and land management. Living in a flat which cost just 14 a month rent and rates - and had no hot water or heating of any kind - made Rennie more keenly aware of the divide between the haves and have-nots. "I had to wash at the old swimming baths in Storey Street." he recalls.
"I'd been interested in some political stuff at school. I used to get agitated about environmental issues and so on. But this was different." He was elected deputy president of the Student Union, and took a year's sabbatical from his course. "Don't be silly, vote for Willie" was the never-to-be-repeated slogan. But when he returned to his studies he found that "cell biology just wasn't the same." So he stood for council election in Paisley Foxbar in 1988.
"The inequalities in housing and amenities were a real shock. I used to go into people's houses in Foxbar, and I'd be horrified. There was mould growing on the walls, they were so damp. I remember one couple who lived at the top of a tenement in Foxbar. They couldn't ever leave the house at the same time, or their neighbours would break in and strip the place. No-one should have to live like that."
Rennie forced the Lib Dem vote share from zero to 26 per cent, and a politician was born.
Despite his land management degree and a further management qualification from Glasgow College, he travelled to Cornwall to work as constituency organiser for Paul Tyler - who had been elected by just nine votes in 1974, then lost the seat to the Tories. But, surely with Rennie's help, he won the seat back in 1992, and Willie Rennie looked set for a behind-the-scenes career in the party executive.
But with a few provisos. "I was determined to have a life outside political organisations," he says. "Because it can become quite removed from reality if you're not careful. So I gave myself till 30. No more." Meanwhile, he proposed to his long-term girlfriend Janet, whom he'd met in Paisley where she was studying industrial chemistry. Luckily she found a job in Cornwall - at its one and only chemical plant, so they married in 1992 and set up home about as far away from their families as it was possible to get on mainland Britain. The following year, the decision had to be reviewed when Rennie's father decided it was time to retire. Would his son be interested in taking over as the third generation?
"I did think seriously about it," he says. "There was a lot I missed in Scotland. But the Lib Dems were on a roll at the time, so I said 'No'. To be honest, I think my Dad was quite relieved. At least he could sell up and retire in peace."
It was 1997 before the Rennies returned to Fife, and bought a house in Kelty. Willie had been offered the job of chief executive for the party in Scotland to prepare for the 1999 Scottish parliamentary elections. "This was a big transition - from being in opposition to being partners in government. And it's amazing how the party changed. There is a bit of a contrast now between north and south of the Border."
Of course, some might say that the change is very hard to detect. That the Liberal Democrats deliberately distance themselves from anything unpopular that the Scottish Executive implements, calling it "Labour policy" despite the coalition.
Rennie frowns. "No, no," he says. "We do accept responsibility for what happens. But we also need to make people aware of what we're aspiring to."
I think I'd probably need to be a politician to understand that; so I asked if he felt the resignation of Charles Kennedy and the fracas that followed had made things especially difficult during the by-election.
"No. Quite the opposite. Charles Kennedy is a really nice guy. It takes real courage to come back from all of that and take part in a very high-profile by-election, and he handled it with real style. He was so relaxed and natural. Really good with off-the-cuff questions. When someone shouted out 'We love you, Charlie' he shouted back: 'Now let's not start a scandal'. When he was asked who he was backing in the leadership contest, he said 'Judging by this reception, I'm voting for Willie Rennie.' He was excellent."
I wonder if Rennie has any anxieties about compromises that may be necessary in the future. Integrity among politicians is a complex issue. The courage of your convictions can - as Tam Dalyell discovered - lead to many dismissing you as intractable or a crank.
"When you get to the position of worrying about the next headline rather than what you think you should do, then your vision is really distorted. You get worried that what you say will be misconstrued, so you get cautious. And before you know it you've turned into an amoeba."
Rennie says this with distasteful precision. One suspects that the biologist in him has recognised all too many amoebas along the way "I don't think anyone gets it completely right. But you have to stay flexible and be open to change."
There will be plenty of that in the Rennie family with the new job. But so far, everything's to plan. "That's because part of the plan is change," laughs Rennie. "When our second son, Stephen, was born two years ago, everything changed; when I left the Scottish Executive, things changed then, too. And spending half the week in London will make even bigger changes, I'm sure. But being back here at the weekends is the core to it all. For my family and for my constituency."
Which means, of course, that he may be running in the Kelty coal race again this year. I wonder where he'll practice.