An amazing 60% of a club runner's muscles will suffer some damage in a marathon with the constant pounding leading to a loss of about 2cm in height.

As Fagan says: "The day after, I couldn't walk down the stairs, and stupidly I didn't rest, so it took me three months to get back to form and fitness."

Butler also has trouble negotiating the stairs but says as an elite runner she is more used to the volume of running.

"It is not unusual to run more than 20 miles in a day's training. So it is not as big of an increase for our body to handle even though we are going that bit faster and for a few more miles," she adds.