To an extent it is about getting stronger and faster, but it`s also about being able to do things with less effort because you do them more efficiently.
Getting into better shape is really a side effect of martial arts training, not the primary goal for, for alot of us. If getting into shape is your main goal, I`d say focus on that end of it and goto a gym. Push some weights, run or swim, and stretch. Boom you`re in shape. But that`s just me I suppose.
What do you mean by "in shape"? Fat loss? Building muscle? Cardiovascular fitness? Flexability? Different arts are going to focus on different things.
Boxing and Muay Thai both produce very ripped up fighters but I don`t know if that`s what you`re looking for. For that matter I`ve seen guys that do Tai Chi Chuan like it`s going out of style, and they have thighs like solid tree trunks and blast-furnace metabolisms.