Watson’s injury isn’t particularly debilitating by the time he meets Holmes (and not even consistently in his leg, Doyle moves it to his arm at one point). And he’s described as thin and athletic (he played rugby and was a military man) at least at the time he first meets Holmes.
Holmes in the stories is a champion boxer and fencer, studies martial arts and is frequently noted to be freakishly strong.
And Downy is actually a decade older then Holmes would’ve been at the time of Watson’s marriage. Law is about the right age for Watson at the same period.