Stiffness

I am no good at sports and other activities due to sitting and staring at a screen for most of my life.  What are some ways to loosen up, get more flexible, and be less stiff?  For example, I throw like a girl in baseball by just using my arm and not the slingshot effect of legs/torso. 

No jokes about asshole stretching, I've already foreseen that reply.

Dear /sports/
I get a huge erection every time I run, which gets in the way as my penis is very large.  I've tried strapping it to one of my legs, but then I can't get out of the 'start' position.  I've tried swimming in my local lake instead, but that's even worse, as I just start dragging weed.  I need to exercise, so what should I do?