He loves to run.
He started as a kid, taking the neighbour’s dog for long runs, glad to escape the confinements of a loveless home. He wasn’t a fighter then, so it also helped him escape when the other boys in school decided they needed someone to beat up again.
He did it all the way through his medical training and of course in the army as well. It sort of became his drug. He was always chasing the next runner’s high, the next rush of endorphins.
He took it up again after Sherlock rid him of his limp.
He goes late at night, because he likes the quiet and (mostly) empty streets of London. By now it’s just running and not a drug anymore, he’s found an even better substitute – Sherlock.
Sherlock used to look at him in a strange way when he came back. And then leave the room.
Now that they're a couple, John dares to ask.
“You look positively ruffled. I always thought that you might look something like that after having sex. You do. Though you smell better after sex.”
“Thanks. I think.”
John peels his shirt off. Sherlock looks intrigued.
“I wonder what you look like when you have sex after you went for a run.”
“Let me just take a quick shower...”