Miles is about 30.
Miles was born to older parents, i.e., more mature and perhaps less flexible in how they raised him than younger, less experienced parents. He was a bright kid, not a genius but certainly at the lower end of gifted. As a kid, and into adulthood, he always enjoyed observing the world around, finding much to amaze him. He was, nonetheless, quite capable of being analytical, a bit removed about people and society. He was therefore never depressed by what he saw. He had a number of girlfriends, but he is still not sure of what he needs in a long term relationship: not sure who she might be, not sure if he wants a life-long commitment -- maybe he's waiting to be older like his parents were.
Just before the time of the story, Miles did triathlons, not professionally -- it was just one of those explorations. Miles tells a story about being hit by a spectator's car in the lake where the swim portion of the half-ironman competition took place. He claims to have finished the race despite getting hit by the car. His event time would have been a lot like his intellect: great for a guy built like him but not athletic genius level, i.e., not professional athlete level.
As the story begins, we don't know about Miles' medical issues, only that he has been cut off from his former life. He has maybe quit a corporate job doing systems which track users of medical services; maybe he has had some thoughts there's something unethical about that, but it's not clear. Life's carnival prizes are already starting to look a little less shiny.
Miles is PTSD throughout the story. PTSD is a metaphor for his new life. In this new life, what he fears seems to be truer than maybe it ought... truer in his head. Miles was probably in pretty good shape at the time of his accident, but doctors did a lot of tests, maybe to be sure, maybe to run up bills, but as results came in for medical tests -- mostly absurd in light of being hit by a car -- we have to wonder whether they are real. For the wrong person? Just absurd?
More generally, Miles cooks, does his own decorating, his own cleanup. He's consistent in how he takes care of himself, casual in his dress, without pretensions, has interests and he values that quality in others, as he will in Margo. He at times seems like a pillar, a rock, even though he's struggling inside to live with his fears, the fears of this "new me."
His thoughts/saying:
-- Dissolve into the flux.
-- As we die, the cosmos is dying too
He means these things tongue-in-cheek:
-- "We will close with and engage the enemy at a time and place of our choosing."
-- "Mission accomplished."