Great job on what you've built so far! Walking's a solid foundation and adding lifting will round things out nicely.
Agreed completely with what fazizzle laid out, and honestly getting yourself in the door is the toughest part by far.
Once you're physically at the gym, everything after that is manageable. Plenty of days I dread heading over there and tell myself, just 10 minutes, one single exercise, three sets, then I'm free to go. Never actually walk out that fast though. End up putting in a solid half hour and feel noticeably better once it's done.
Starting out, I'd suggest a routine hitting your whole body, done a couple times a week using weights lighter than you'd expect to need. This gets you comfortable with the movements, keeps injury risk low, and touches every muscle group regularly. A trainer's ideal for learning technique, but if that's not in the cards there's no shortage of free tutorials online covering form.
Been lifting three decades myself, on and off, and picked things back up two years ago using a tracking app that's free to download.
