Are you a metal bassist? I've seen some of those guys and I swear they hang their damn instruments down below their knees(I'm looking at you Robbie Merrill
), heck I'd like to know where in the heck they can find a guitar strap so long.
You could shorten your strap to be at the same height standing as when you are sitting, it's a perfectly viable option. Another possible solution is that you you could just keep it at the same length and practice a lot while standing(if your fingers are long enough), and keep stretching those tendons. I practice standing up myself and it's to the point where I am actually more comfortable playing that way.
Recently I decided to learn Hey Man Nice Shot, and I swear I've never seen a song with so much hammering before. I play it with a five string bass so I'm hammering on as far up as the fifteenth fret on the B string and it's taken me a few days to actually get through the whole song.
I find that bass playing is very much a physical activity, almost athletic really, it requires strength, stamina, and dexterity.