Close to half of the competitors in this year's Spine Race - a gruelling ultramarathon from Derbyshire to the Scottish Borders - have dropped out midway. It's a story Kim Collinson knows well.

An ultramarathon is any race longer than 42 kilometers. Races can range anywhere from 50 to over 4,000 kilometers. But one of the more common lengths covers about 100 kilometers and takes ...