Speaking to Radio 1 presenters Matt Edmondson and Mollie King at the start line, Greg thanked banner-waving supporters for coming out.
Greg described the challenge ahead of him as “ridiculous” but said he would “give it everything”.
“You’ve got to commit and I’m going to commit,” he added.
“It’s one of those things that needs to be a challenge.
“I’ve had a big breakfast, and I’m ready to go.”
Wearing a red nose, Greg punched the air as he cycled away from the cheering crowd and headed off towards Bristol, the checkpoint for the end of day one.