That's tough. I have found that I can't exist as the human I want to be if I don't do something every day. Doesn't have to be much . ..just stretches, planks and pushups before breakfast is enough to keep me operational though not fit. Injuries are the big bugaboo. You get some little thing like that so then you can't do something and that makes it worse and it's a downhill slide. It's tough. I had cataract surgery on one eye at a time, so 2 weeks in close succession during which I was forbidden to lift more than 20 lbs. or do anything that would make me sweat. So then I got sciatica just from not exercising. Almost got that fixed but wow am I weak. Back at it now. I like to do fast walks as well as cycling coming back from this sort of thing. So walk until the arm is better, like 3 brisk miles every other day and then every day when that gets too easy. You can probably plank just fine. Planking and pushups after stretching every morning go a long way toward injury prevention.