Marathon
The hardest part is remembering to slow down for normal things.
He can't chow through two lunches at work. He can't use his speed to run assays. If a piece of evidence needs to be overnighted to Gotham, he can't just run it there himself.
He has to make himself take individual steps on the sidewalk, to stand in lines, to wait for people to answer the phone. He can't dash up the stairs instead of waiting for the elevator.
Everybody says the Flash is just "a regular guy," but no one knows how hard he works to stay there.