We are the same age and I have workout much of my life. Full extension is better. Just don't go to the point that the weight transfers too much to your tendons.
I find at our age the amount I lift is not my goal anymore. I do the reps slowly over about 3 seconds a rep. The best is to do the big muscles (thighs, chest and back) for a while and get use to working out. Then add arms, shoulders and calfs.
I assume you have been cleared to do weight lifting. A trainer would not be a bad idea to get it down. Learning how to breathe as you lift ( don't hold your breath) exercises to avoid ( most cardiac patients are told to avoid squats) and someone to check your form so you don't get injured. I am still able to add muscle ( not as fast as my 22 yr old son
) and keep the fat at bay.