Fenwick will carry the flame as Albert Square's Billy Mitchell in a live scene to be slotted into an upcoming episode of the BBC soap.
His alter ego will narrowly make it back in time from a football match to fulfil his life ambition on July 23.
"I've come in of a morning and do a few laps of Albert Square," he told the Radio Times of his intense preparation.
"I basically want to find every pothole so the crew can fill them in. I've started thinking, 'God, it's not far away now'."
The actor went on: "I've been having these bizarre actor/athlete dreams where I've got no trousers on or I'm tripping up. It's a weird blurring of fiction and reality.
"If I can achieve that on the night and do a little bit of acting on top, then I'll be thrilled. Hopefully, there'll be a large gin and tonic waiting for me in my dressing room afterwards."
Fenwick added: "The fact that an extra couple of million people will tune in to see whether I fall over is a pressure in itself."
Details of the special storyline first emerged last November. EastEnders airs Billy's Olympic episodes on Monday, July 23 at 8pm and 9pm on BBC One.