Terminal cancer sufferer Jane Tomlinson is struggling to complete her 4,200-mile charity bike ride across the US, her husband has said.
Temperatures have reached more than 38C (100F) during the ride
Mike Tomlinson said the mother of three from Leeds was using every reserve of her strength for a challenge that seemed "incredibly futile" at times.
She is suffering from dehydration and extreme fatigue and has had back pain since before the ride began.
Mrs Tomlinson, 42, is trying to cycle from San Francisco to New York.
She is joined on the 62-day trip by Leeds Metropolitan University lecturer Ryan Bowd, 27, of Calgary, Canada, and 40-year-old Martyn Hollingworth from Huddersfield, West Yorkshire.
In the latest stage, the trio covered 84 miles over three mountain passes in temperatures of more than 38C (100F) as they left Nevada and entered Utah.
As the ride was stopped for the first time by a storm, Mr Tomlinson, 45, said: "Jane was quite emotional at the finish and sat crying wondering what she was doing in the States, and just what was the point of cycling across it.
"It's something that none of us can help with and I can understand why she feels dispirited.
"When you have a short time to live it's a huge sacrifice to dedicate it to try and improve things for others.
The ride began at the Golden Gate Bridge in San Francisco on 29 June
"This ride is taking every reserve of Jane's strength and it seems so incredibly futile.
"I think we all have bad days, but for Jane she has the pain of an aching and painful body.
"I think Jane will continue but I wouldn't blame her if she was on the next flight home from Cedar City."
Since being diagnosed with advanced metastatic breast cancer in 2000, Mrs Tomlinson has embarked on a series of endurance challenges that have raised more than £1.25m for various charities.
She is hoping her latest challenge, which is due to end on 31 August - the sixth anniversary of the day doctors first told her she had just six months to live, will take the total raised past the £2m mark.