John Bishop has set off on the first of three marathons on consecutive days, hours after completing a gruelling cross-Channel row.
The comedian said his body had been taped up to ease crippling muscle aches as he set off in front of well-wishers from Dover, Kent, as part of a "week of hell".
The comedian was averaging 10-minute miles for the first hour of his run and after taking an early tumble, but quickly regained his stride.
Earlier, John battled sleep deprivation and became "delirious" during the 26-mile row across the English Channel on just one hour's sleep, after earlier completing a 185-mile cycle ride.
With fellow rowers Davina McCall, Denise Lewis and Freddie Flintoff, he completed the row in seven hours, 45 minutes, almost two hours longer than expected, for the BT Sport Relief Challenge.
As he continued on the run, John found he was suffering even more pains when his running gear began to rub his skin and he had to resort to unguents.
Taking a lunch break in the village of Bridge, near Canterbury, in Kent, he said: "I've done around 14 miles so far and I am chafing.
"I should become a spokesperson for Udder Cream. I'm using a whole different muscle group today from the cycle and the row but I am already feeling it. The only problem with running from the coast is that you have to go uphill."
The aim is to raise enough money to vaccinate 250,000 children in Africa, as well as help with projects in the UK and the comedian hopes to make it to Trafalgar Square in London on Friday. The celebrities' efforts have already raised more than £400,000 for Sport Relief.