In reply to notaclue:
Of course there is a benefit to upper body strength. Stops you from doubling over when you get tired. As Marek says, core is important. For running with a pack shoulders are also important. Press ups are one of many exercises, but not much use on their own.
Isolating muscles body builder style is also not a good idea.
I do Yoga for static core stuff, which can be a surprisingly tough strength workout, and more explosive body weight exercises in tabata interval style.
I also have a fitball to do all sorts of stability stuff on, but have to admit to being a bit lazy with it and not getting on it recently. Press ups with the legs on the fitball for example would be much more useful than traditional press ups.