After donning the giant 9ft-long snail suit, Lloyd Scott had to lie on his stomach and slide along on a sled until he'd completed the famous 26.2-mile course.
The 49-year-old from Essex - raising money for Action For Kids - suffered nosebleeds, vomiting and cramp as he crawled along for eight hours per day.
At one point he was rushed to hospital to have the blood vessels in his nose cauterised because his face down position had caused continual nosebleeds.
After finishing his 'race' outside Buckingham Palace, Lloyd said: "This has been much, much harder than I thought it would be."
"I've also had nosebleeds, I've lost a toenail, and the pain in my shoulders and neck is constant. But I'm not giving up as too many families are relying on me."