To get back into moving I took up team sports, specifically soccer. Two seasons in the local adult rec league each year (winter and summer) with 8 regular games + 1-3 playoff games each season. It's only once a week, but it got me started. After that I added in yoga for back health (combatting chronic sitting syndrome and then an actual back injury caused by a careless driver). After the back healed up I added in running, now 2x5k runs each week. Then for added fun and stress relief I've taken up Brazilian Jiu Jitsu (on pause due to a soccer injury acting up).
The main thing to remember: If it hurts, stop and consider why. When I started up running for real last year I was going 3 days a week. I still weighed over 200 pounds and my knees were in poor shape after the third run. I cut it down to two days and they stopped hurting entirely (beyond general soreness or the effects of age). With BJJ, I noticed my shoulder (the soccer injury) acting up. I started cutting back on certain movements, having my training partners focus on my left side instead of my right. Took a week or two off. The pain remained (actually got worse) and I went to the doctor. Got a steroid shot, resting for a couple more weeks and then I can get back to it. Rest when your body needs rest. There are folks at the gym I attend that work through their pain and injuries, and they're chronically injured (though in different places each month). A bad shoulder like mine means I'm going to do some movements wrong and may injure my hip or my leg trying to compensate for the lack of strength on my right side. I can still run and do lower body and core exercises that don't require me to use my right arm, so I will.
Routine: Routine makes a big difference. Even if I can't make my jiu jitsu class, my running is baked into me now. Tuesdays and Thursdays I get off work and hit a trail near the office. I can't imagine not doing that now (especially with this spring weather, it's fantastic).
Partners and classes: On my own I'm prone to skip, I have little motivation to do things for myself. I'm running with a friend of mine and if I don't go I feel like I'm letting him down (especially since, at this point, he needs it more than I do). Classes are similar. I don't really feel like I'm letting down my instructors or classmates, but I know I'm falling behind my peers (those that started around the time I did) by skipping. So I'm strongly motivated to make it at least 3 nights (it's offered M-F) a week. In part for the sense of competition, but also so I can be useful in practice with my classmates instead of being chronically behind.