dont worry silenscer, with an instrument and from your age(with much more room to grow taller and stronger that is) plus the fact that we can always get used to whatever situation that we can be in(adapting to changes thru the correct repetitive practice), training up for physical fitness to play geetar is actually a minor factor we should be looking at.
Among the reponses, theres mentionings of getting a thicker cushioned and wider strap, its actually one of the more useful suggestion.
if we are looking from the point where standing and playing a guitar/bass, the pressure we have to endure most, other then the weight of the instrument itself, is the area on our shoulder where the strap will be resting on.
Usually this is the area which can give a lot of problem to the tiredness and aching if the instrument is on a heavier side, with a tiny strap and through hours of playing the instrument standing up, with bad posture.
instead of worring bout physical fitness, we shud be looking at good strap, correct playing posture(playing the instrument with strap and standing up) and also the amount of time spend on playing(anything when overplayed, will result in pain and perhaps injuries)
imho