Farah, who won gold in the 5,000 metres and 10,000m in the past two Olympic Games, admitted his second full marathon had taken its toll in a race where there was a world record pace at the halfway point.
He told the BBC: “I am knackered. The guys went for it, they were on for world record pace, so it was do or die. I went with it and hung in as much as I could.
“It’s so different to the track. It’s incredible. It’s different pain, different training but I’ve really enjoyed it. I gave it all, 110 per cent as I normally do.
“I’ve got a long way to go in the marathon. You get heavy legs. Mentally you’ve just got to be strong, take your drink and just pace yourself.”
Eliud Kipchoge of Kenya won the 2018 men’s race, with the 33-year-old making it a hat-trick of victories after his first-place finishes in 2015 and 2016.
Ethiopia’s Shura Kitata Tola came second ahead of Farah.
Olympic 5,000-metre champion Vivian Cheruiyot of Kenya won the women’s race at the second time of asking, completing the course in two hours, 18 minutes and 30 seconds.
Her compatriot Brigid Kosgei was second, one minute and 42 seconds further back. Ethiopian Tadelech Bekele came third.
There was a home success with David Weir winning the men’s wheelchair race for an eighth time after a sprint finish.
The 38-year-old Weir clocked 1:31:15 to beat Marcel Hug of Switzerland into second, while Daniel Romanchuk of the United States was third.
Weir said: “The end was tougher than last year. Mentally and physically, I felt better coming into this.
“At the beginning I felt a little bit nervous to be honest, it was a little bit hot and that’s why I lifted my visor up to get some air and to cool down a bit, but I’m really happy this year.
“I know what I’m capable of and I’m going to do the rest of the marathons in the autumn as well and race the guys.
“I’m just enjoying life to be honest and I think that’s what’s helping.”
Madison de Rozario of Australia won the women’s wheelchair race for the first time ahead of four-time champion Tatyana McFadden, whose fellow American, Susannah Scaroni, was third.
Lily Partridge, the first British woman over the line in the elite women’s race, with a time of under two-and-a-half hours, said: “It is unbelievable. I felt absolutely fantastic until 35K (21.8 miles) and then it started to bite. And then it got slowly worse.”
Partridge, who claimed eighth place, added: “It’s only the second time I’ve run past 23 miles and I’ve only ever done the full distance once (during training).”