You can also use large rubber bands sold at sporting shops. Did someone already mention this? I use them like the equalizer. They do more than just strengthen arm muscles. They strengthen your entire tortional motion from knees to hips to side abs, to deltoids, to rotators, to wrists, to fingers. Now this may not make your body turn faster (which is more a technique issue), but these exercises will help hold your body together and keep you healthy, especially as you age.
An advantage of the rubber bands (I think they are called TheraBands, or something) is that you can also exercise your side arm throws. Trap your two throwing fingers into the band and mimic a side arm motion.
You also put the bands on top of a door (I tie the bands off into a large 5 foot circle for all of these exercises), sit on a therapy ball, and do lat pull downs, or standup and do rotator muscle exercises for inner and outer rotators (good for sidearm motion--just ask any pitcher who no longer pitches because of rotator injuries).
I also stand on one end of the band and work on biceps, and other motions for the flyfishing that I do.
Then stretch it all. Disc Golf isn't easy on the body. It is harder on your body than ball golf, IMO, which is why it is good to fling your body around 180 degrees on your throws to relieve all the pressure (which typically isn't done in ball golf).
Oh, I almost forgot, you want longer distance, too. Like Blake says, Work on your snap.