Define "in shape." Round is a shape.
Seriously, I do not think you have to be some sort of Greek god to do this art. Two of the six ninja the shogunate sent to Shimabara were in their 60s. If you can't do a technique at that age because everything relies on muscle, it is not worth studying. A friend of mine (non=Bujinkan) said that you should study as if you were 70 becasue hopefully you'll be that age someday and then what are you going to do?
Indeed, I have seen a few people plug holes in their technique by using muscle. If you have it, its great. But if you meet a bigger guy, or are tired, or get old, what do you have to fall back on? I have heard that Nagato lost a lot of his old muscle mass on purpose because he felt he was relying to much on it instead of good taijutsu. He also wanted to build up more endurance muscles rather than the snap type he used as a kick-boxing champ.
But on the other hand, I was talking with some guys visiting Japan a while back and the subject of a certain member of the Bujinkan came up. (Note- he is not registered here at Martialtalk.) The guys brought up the point, "how the heck can you call yourself a martial artist when you look like the Staypuff Marshmellow Man?"
We practice this art to avoid violent deaths and live long, fullfilling lives. So why are there so many martial artists that look like they are a decade away from triple bypass surgery? I have seen guys come into class that are just plain fat, and then they sit down and start munching on potatoe chips, chocolate cake bars and such while waiting for the teacher.
Staying within a healthy weight is not a matter of some secret technique. You just put the damn Twinkies down. You take walks and use the stairs from time to time instead of the elevator. You use more calories than you consume. Even if it takes a long time, you can get to a healthy weight.
My mother in law has to check her insulin levels now, and she has never been what I would call fat. There are so many health hazards that can be avoided if we just watch what we eat. It is silly to do pushups and learn things like how to snap necks on the excuse that we need to do so in order to survive to a ripe old age and then not even bother to check our cholesteral levels at least once a year.