The 29-year-old was last to finish in a 51-strong field in a time of four hours, 15 minutes and five seconds in hot conditions.
But King, who walks for Colchester Harriers, is just delighted to be able to call himself an Olympian and will take away great memories, despite his lowly finish.
"Hopefully, what people will have seen today will encourage people to take up the event," said King.
"It is a tough event. The sport does not get much coverage, but this will go a long way to putting it on the map. Hopefully, people will be inspired by what they have seen today.
"I'm going to put my feet up for a few weeks before deciding what to do next. It's only really starting to sink in that I have taken part in the 2012 Olympics. I'll remember this for the rest of my life.
"It was pretty hot out there and it was essential to get my drinks strategy right, to make sure that fluids strategy was correct."
King made the most of his time in the spotlight, and wanted to share the moment with his fans, despite being physically shattered.
He said: "I didn't realise how far I was from the finish, so when I started to high five people in The Mall my arm soon became pretty tired with it all.
"Not everyone got a high five, but I really appreciated the support."
"I'm feeling much better now I just had to put my feet up. I had too much blood in my legs and I was feeling a bit dizzy. There were a few casualties along the way today, but I feel fine now."