I am no expert, but at the gym I went to they had about 12 different machines. I started on the first one, did about 10 repetitions with the weight I was comfortable with, did the same all the way through the 12 machines, Every so often I would try to add a few more repetitions or add weight, until I reached what I thought was safe. I did not and I repeat did not try to keep up with the young ones, I took my time and did not try to strain anything. I was there to build my endurance, not impress anybody. I skipped the treadmill, my legs and balance are messed up, the bikes
When my knees started to give me feedback I slowed down on those machines. My right shoulder is also real cluster from 18 years of walking with a cane, so I also made sure I did not strain it.