Terminal cancer sufferer and fundraiser Jane Tomlinson said she felt more fear than excitement ahead of her 4,200-mile charity bike ride across America.
Jane has been suffering back and hip pains in the run-up to the ride
The mother-of-three from Leeds started her toughest challenge yet, from San Francisco's Golden Gate Bridge to New York, on Thursday.
But just weeks ago she completed her latest course of chemotherapy and has been in pain in the run-up to the ride.
"I'm just living my life the best way I can," she said.
The Ride Across America is due to end on 31 August - the sixth anniversary of the day doctors first told Mrs Tomlinson she had just six months to live.
So far, her fundraising challenges have raised more than £1.25m and she is hoping this latest test of her endurance will take her total past the £2m mark.
Before she set off, Mrs Tomlinson said the pains in her back and hips had left her feeling a bit daunted by the ride.
"I really would like to feel really excited about the prospect of the nine weeks crossing the US," she said. "But at the moment it's fear rather than excitement.
"I'm hoping that over the next couple of days my hip and back will settle down.
"To be feeling ill now is a disappointment, but we will be on the road and we're hoping things will improve."
The 42-year-old will be joined on the 62-day ride by two friends - Leeds Metropolitan University lecturer Ryan Bowd, 27, of Calgary, Canada, and 40-year-old Martyn Hollingworth, of Huddersfield, West Yorkshire.
Along the way she is expecting to endure temperatures of more than 38C (100F) and altitudes of more than 11,000 ft.
But she said she would not have contemplated her latest fund-raising challenge without the support of her family.
Her husband Mike, 45, will support the cyclists along the route with their nine-year-old son Steven while daughter Rebecca, 18, will join them when she finishes her A-level exams.
"If they weren't here I don't think I would be doing this," Mrs Tomlinson said. "They're here so we can be together throughout."
Mr Tomlinson said his wife would take the ride "one day at a time" and would rest if she felt unable to ride on any day.
Last year, Mrs Tomlinson announced she was retiring from charity work after completing the gruelling Florida Ironman contest.
But, following a course of chemotherapy, her health improved and she went on to complete the New York marathon last November in five hours 15 minutes.
Since being diagnosed with advanced metastatic breast cancer, she has also completed a 2,500-mile bike ride, from Rome to Leeds, as well as a number of triathlons.