The man who celebrated his 22nd birthday last week will ride up and down Black Mountain - one of the toughest climbs for cyclists in the territory - up to 38 times on December 14 in an arduous test of endurance that could take 10 hours or more.

In a back-of-the-envelope calculation, Johnston has worked out that climbing Black Mountain 38 times is the equivalent to climbing Mt Everest.

He hopes to raise $10,000 for Canberra Hospital's oncology unit, which helped save his life in 2009 and 2010 during his year-long battle with testicular cancer.

''I've always wanted to do something to pay them back,'' he said.

At the time he was diagnosed he had just been picked in the Australian cross-country mountain biking team and was about to compete in the world championships. He underwent urgent surgery to remove the cancer and still competed, although the operation badly affected his ability to race.

Any will to compete left in him was stripped away by 12 months of chemotherapy.

''It ended my time as an international mountain biker,'' he said.

But he does aim to compete in the Commonwealth Games next year and he has shown strong form lately by winning the 112-kilometre CamelBak Highland Fling in the southern highlands.

Johnston is one of the fastest riders in Canberra according to the online performance tracker Strava.

It shows he is the fastest person up Black Mountain at 7 minutes, 37 seconds and the quickest person to ascend Mt Ainslie at 8 minutes, 45 seconds.

■ Go to repsforareason.net to find out more and donate. Anyone can buy a lap from Johnston for $50 once he has completed 20 reps and ride it on his behalf.