Gaining weight and restoring reasonable muscle mass at 60+

It's never too late to start! I turned 78 in December and been in weight training since my late teen age years. As you get older the change of routine should be lighter weights and higher repetitions. In addition, listen to your body and don't force movement that is painful or may damage past injuries. As we age it's not how much you can lift it's how well can you lift the weights that you can handle for 15 - 20+ reps. And good weight training, at any age, is to not work the same muscles every day - give them a break between workouts. I find that at my present age I have more strength and energy if I have 2 days off between my weight training. And don't forget the cardio!

Something I am noticing in this 76 year old body is that I can injure myself so much easier now. Tendonitis seems to be the malady of the day. First I got it in my hip because I think I didn't pace myself with the running and didn't take days off for a month because I had this great idea that I could start running every day like I did in my youth and everything would just take care of itself. Well the tendonitis took me out of action for many months. Recovery takes a lot longer now. Then I decided that I'll just focus on my upper body, and I set a goal of getting to 10 pull-ups. I slightly modified my frequency to every other day, thinking that a day off in between should offer the interval rest required. I got to 7 pull-ups and suddenly my right elbow started hurting. The next day the left elbow followed suit. I'm almost certain it's tendonitis. I'll see my local doctor today for confirmation, but whatever it is, boy did it decimate any upper body work I might do.
So finally I realize that strength and endurance training at this slightly advanced age of 76 is not like anything I have ever experienced. I may have a glimmer of a process for running and walking and flexibility and balance, and I believe if I could only understand the body's new limitations I could even build strength, but I just don't know how to proceed. Has anyone else had this particular issue and found a way to work with it or am I just whistling dixie to try for such improvement?

I could have wrote what you said! I was so shocked when it seemed that suddenly when I turned 70, I went from being a gym rat, to hurting myself closing a car door. My shoulders hurt so much every morning that I want to scream when I move my arm. Then my elbow got tender from doing bicep curls. Then my knee from going down a flight of stairs. Working out has been my life, until now. So I follow Will Harlow on YouTube, Thriving after 50 and now I have a bit of hope. The adjustment to doing so much less has been long and hard. But at 72, necessary. I wish us both luck with getting back some muscle.