Mavis Paterson, who lives near Stranraer in Dumfries and Galloway, set out on May 30 from Land's End and aims to reach John O'Groats on June 21.
Mrs Paterson stopped at Chatelherault Country Park on Thursday where she met the team from the local Macmillan Information and Support Centre in nearby Hamilton, South Lanarkshire.
Mrs Paterson hopes to raise £20,000 for Macmillan Cancer Support, a charity she started to support after her mother and sister died from the illness.
She said: "The beginning of the challenge down in Cornwall and Devon was a real test, I've never had to tackle hills like it, but I had Heather with me and she has been a tremendous support and we made it through.
"I am doing this for my children, the unbearable grief I feel when I'm sat at home with nothing to focus on is too much. I needed this and it's been amazing. The support we have received has been overwhelming, I never expected it.
"The donations from people I don't even know and the lovely messages, I can't wait to read them all when we are finished the challenge and I have time to look over them all. I really can't thank people enough, what is happening is amazing.
"We have now raised just almost £17,000 and I honestly can't believe it, to reach the £20,000 by the time we reach John O'Groats would be great."
The 81-year-old is undertaking the challenge with her cycling companion Heather Curley, 55.
Mrs Paterson's three children died within four years of each other while all in their 40s.
Her son Sandy died of a heart attack in 2012, Katie after suffering viral pneumonia in 2013 and Bob in an accident in 2016.
Craig Tobin, Macmillan project manager, said: "The team had to come down and cheer Mavis on, she's captured the hearts of everyone and she is truly inspirational.
"To see her taking on a challenge like this, to raise money that will help people living with cancer, people that we help every day at our services is amazing.
"We simply couldn't continue to do what we do to help people living with cancer to live life as fully as they can without people like Mavis who go above and beyond to fundraise."
Donations can be made here.