Welcome! I'm also in my 50s.
The best suggestion I can give is to take a look at the programs Foundation and Foundation Light and start with either of these. I do a lot of stage combat, and the first rule of stage combat applies - safety first. If anything hurts, ease up a bit. It's OK to take longer breaks than suggested - I do all the time because I'm busy and working from home. My own goal is to stay active all day and keep increasing muscle mass. The drawback to working from home is the easy availablility of food and a lot more sitting.