Tom Harrison is going to new lengths in London to raise money for gorillas.

Although the London Marathon started five days ago, Harrison is not quite finished yet.

He's not a slow runner. But he is a slow crawler, particularly wearing his gorilla suit.

The man known as Mr. Gorilla is raising money for the Gorilla Organization -- so far more than 22,000 pounds ($28,500) have been pledged.

He's hopeful of finishing the race Saturday, with his two sons awaiting him at the finish line. They will also be wearing gorilla suits.

He ran the marathon the conventional way last year to raise money for gorillas, but decided this year to try something harder, and potentially more lucrative.

He's developed several different techniques, none of them pain-free. Kneepads brought little relief, and were ditched on the first day.

"It's shoulders, hamstrings, glutes, quads, from the constant crouching," he says. "We're not really designed to do that."

Harrison says the money will be used to protect endangered gorillas in Africa by sponsoring ranger services and helping people living near gorilla habitats to be more self-sufficient.

The Associated Press contributed to this report.