Mick Greener, from Nottingham, will be doing his fourth London Marathon on Sunday.
The 29-year-old teacher said he hoped to run the race in about two hours and 45 minutes, which would give him time to “to get a shower and get up to Wembley for the game”.
“Obviously, I’ve had the marathon in the diary for a while and when Forest made the semi-final and it fell on the same day, it didn’t click,” he added.
“My first thought was ‘that’s annoying’ because I wanted to be in Nottingham for the game, not in London.
“Then I thought I just might be able to get tickets, and I did, so it’s looking like it will be a great day.”
Andy Aiston will also be running the marathon before joining other Forest fans heading up Wembley Way.
The 30-year-old, from Giltbrook in Nottinghamshire, said he was shocked when both events were set for the same day, but is hoping to complete the race in under three hours to give him as much time as possible to make the match.
“I hope it’s going to be a brilliant day for me and for Forest,” he said.
“It could either be the best day ever [if] I do the marathon and then Forest win, or it could be absolutely tragic because the marathon goes completely pear-shaped and Forest lose.
“It feels like a bit of a gamble really but I just have to do it really, given that I’m already there.
“My target is to do it in under three hours which should mean I get over the finish line at about 12:30 or 12:45 so I’ve just got to basically get all my stuff, see my wife at the end of it, [and] sort of enjoy the moment.”
