The brothers will be competing as a two-man relay team in the ultra-endurance Race Across America to raise money for University Hospitals Dorset.

Funds raised by the pair will be put towards new state-of-the-art tilt-tables, which will help physiotherapists provide safe and effective early rehabilitation to some of the most unwell patients in Dorset.

Jack, 33, said: “One of our closest friends suffered a stroke, so we decided to take on this challenge to help give patients in a similar position the best possible chance at recovery.”

He and Alex will cycle 3,175 miles from California in the west to Maryland in the east.

Covering 400 miles a day, they will cross 12 states while climbing more than five times the height of Everest; all within an official time limit of nine days and 12 hours.

The pair will dip their bike wheels in the waters of the Pacific at Oceanside on the southern Californian coast before heading east through the Mojave Desert and across Death Valley into Nevada, Utah and the Rocky Mountains.

Co-owner of the E-motion Fitness Hub in Poole, and father of one, Jack added: “We are excited and nervous in equal measure. The tension is there and we do have concerns about the heat.”

The route will take them through Kansas, across the Mississippi River, into the Appalachians and finally to Maryland where they will reach the Atlantic coast on the dockside in Annapolis.

Once the clock starts, they will take turnabout sessions – one cycling, the other resting – until the race is done.

“The first two to three days will be extremely challenging as we climb through the desert to the Rockies in intense heat. Temperatures in the desert could be up to 40 or 50 degrees C. We’ve never experienced that kind of weather before. But then, after we are through the Rockies, we have a 1,500-mile descent,” said Jack.

Travelling in a van with the pair is a support team of friends including a mechanic, driver, navigator, videographer, cook and ‘dogsbody’. Everyone will sleep in the van including each brother when resting.

Jack and Alex have cycled together before including on a 1,000-mile, London-to-Nice ride for a cancer charity.

Months of training will culminate in a final 24-hour practice ride in Dorset before they fly to the US ahead of the start of Race Across America on 18 June.

Alex, 32, a former Royal Marine who is a father of two and lives in Bournemouth, said: “Our preparations are going well. We’ve both got full commitment to this. We just want to get on and do our best.”

Funds raised by the brothers’ bike challenge will contribute to the cost of two robotic tilt tables.

These will help physiotherapists care for very weak and sick bedridden patients, including stroke patients, those in intensive care who need rehabilitation after being removed from a ventilator and patients with spinal injuries.

Stroke service lead consultant Becky Jupp said: “Prolonged horizontal bed rest can lead to loss of function and weakness in the lower limbs, as well as cardiovascular damage. A tilt table enables patients to start safe, early rehabilitation in a vertical position far more quickly than they otherwise would. This reduces the amount of time they need to spend in hospital.”

Karen Smith, fundraising manager at University Hospitals Dorset NHS Charity, said: “The funds the brothers raise will make a hugely significant difference to people’s recovery from severe neurological damage. What the brothers are doing is an absolutely incredible challenge involving immense physical pressure on them both. For one thing, they will experience every type of weather condition as they cross America.”

Jack said: “We are determined to complete the challenge so we can help the charity fund this incredible equipment which will make a huge difference to the lives of stroke patients at University Hospitals Dorset.

“We are also doing this race to show people that if you apply yourself then anything is possible.

“If you have something you want to achieve, you can do it if you take the right action and have the right mindset.”

Asked about the brothers’ relationship and the support they have had from their families, Jack said: “We have not always done cycling and certainly nothing on this scale before. We are brothers and of course we are close. That’s until we don’t get on!

“On the race itself, we won’t really see each other much for the nine days. As it’s a relay, there is only one bike on the road at a time so we’ll only see each other for a few seconds between each other’s sections.”

Alex said: “Our families have given us fantastic support and put up with our many months of training to get to this stage.

“The donations and messages of encouragement we have received so far mean so much and will spur us on as we push ourselves to the limit so far away from home.”

Among Jack and Alex’s supporters are team members at the Poole-based property finance lender MSP Capital.

Becky Harris, finance director, said: “Jack and Alex have set themselves a truly phenomenal challenge and we wish them the very best as they take to the route. We’ll be tracking them online all the way and sending them our support.

“Their initiative is in aid of amazing medical technology that will make a hugely positive difference to so many people here in the south of England now and in the future.”

For more information on Race Across America and to sponsor Jack and Alex, visit https://gofund.me/df30a360.

To find out more about the work done by University Hospitals Dorset NHS Charity, visit UHDcharity.org