Bou, I think Smith's still being fit for service is to do with a number of factors/ Firstly people are living a lot longer now than we would have done a hundred years ago, so by the time the story is set 70 might be the equivalant of the 50's now.
Plus there's bound to be medical improvements over the intervening years, so that will have helped keep Smith fit.
Add onto that the fact that in the intervening years Smith has been working in a menial job as opposed to retiring/working behind a desk that would contribute to his ability to ruck.
As would the fact that he'll have been in combat training since the war began. If the fight had taken place just after the war began then it might have been less believable, but seems okay now.
Oh, and finally, the fight was in a gravity bar, so the conditions for a hand to hand would have been better than if he was somewhere else.
Personally I think it's not something that needs explaining within the story.