Yep, I'm with Mr. J on this. It'll drive you nuts, but your first priority is not making it worse.
Some things to remember:
If you're, say, under 25, they say that if the busted bones are in the same room they'll heal okay. Well, that may be, but you're 50 now, and healing will be slower, especially considering where it broke. Be gentle with it. Healing it right is priority.
Next, you don't want to go into full couch potato mode. A good walk will do just fine, without futzing up the busted bone ends. Take that from a man with a crooked hand, 'cos I didn't.
Aerobic activity, fine, say on a bike, a stationary bike, but walking is best. Gentle aerobic activity, zero impact stuff. Don't worry about being 100% fit, just don't lose a lot of ground.
Eat right. Plus drink milk, the benefits of that are obvious. Stay clear of the alcohol... mostly because when a little lit you may be tempted to do something dumb which may reinjure the bad arm or something brand new and different. Guess how come I know that one....
Do some dumbbell exercises on the uninjured arm. The body is a funny thing, it wants to stay symmetrical, so if you work the uninjured arm some the injured arm will get some benefit too.
When the cast comes off, you will have some very stiff joints, 'cos they haven't moved for a while. Range of motion will be less too, temporarily. It'll come back with a little work. Gentle stretching is good, but again, don't overdo. The cast comes off usually as soon as it can, but the bone still isn't back to 100%, and you can still do damage.
Catch up on your reading and such. I'm not gonna get all warm and fuzzy on you, but if you keep your mind occupied you'll heal better and tolerate the enforced inactivity better. Plus who knows, you might discover something like the Patrick O'Brian Aubrey-Maturin series and develop a lifelong addiction.
Hope that's of use
doc...