Dr Andrew Murray, a GP from Edinburgh, had to give up the challenge yesterday after suffering stomach pains, dizziness and dehydration as he climbed Mount Cotopaxi in Ecuador.
Dr Murray wrote in his blog: "Running 50km at altitude, running off into the bushes every couple of kilometres was something I was dealing with, but I just could not keep any food or fluid down and became dehydrated and had the head spins.
"Although the volcano and most of the very high altitude was behind me I was in no fit state to run another 100-odd kilometres."
Updating followers later, he said he had "managed to eat a whole banana" but had lost six kilos in weight during the previous day's run.
Dr Andrew Murray had described the run - equivalent to completing four marathons in just 24 hours - as his greatest challenge yet.