Old Habits Die Hard
At first it was small things: bad sleep he chalked up to his poor sleep schedule, consistent stress even when there was nothing to stress over that he reasoned was just paranoia, chronic fatigue that he brushed off as getting older.
Then it was stuff he couldn't rationalize away: getting winded too fast walking down stairs, having trouble carrying a box of files to his desk, and most obvious the onset of weight gain.
He hadn't really thought of it before but it did make sense. When he was Shroud he was constantly moving, rearranging the hideout, tracking down leads for the pulse. Even in prison he had resorted to working out as a means to cure his boredom. But now he worked an office job, not much exercise can be done with that line of work.
After realizing what was happening Elliot got a bit self conscious at the reality he was getting physically weaker and slower. He decided to put a stop to that and bring back one of his old habits, training the way he did in his 20s. Lets just say he isn't the only one appreciating the payoff.😉