Hi, Junkbuyer. I’m 70 and have been Fasting for almost 6 years. Unless there is another issue at play, you can regain your strength and build muscle mass. Work up slowly. Your ‘Fast Strength’ sounds like the app I use called “7-Minute Fitness” — similar exercises. Don’t do the jumping jacks or the running in place or other high impact exercise. Do the others — even if you do only 2 in the time allotted. If this week you do 2 seconds of plank, next week you might do 3 seconds. BTW, lunges are a slide, not a leap forward, right? Triceps dips are worth a try.
I started to do the 7-Minute Fitness about 4 years ago and have increased my strength markedly. When they ask you to do one of the exercises you can’t do [high-impact ones], substitute another one. Target the upper body. Hold a 16 oz can of whatever from the pantry shelf in each hand and do bicep curls. Or keep your arms straight and raise the cans over your head. Depending on how many you do and how often you do them, you will find that your strength will increase.
You seem to have the cardio and endurance exercises down pat. And your legs must be fit from the swimming and walking. You could even exercise your upper body when you swim!
Good luck.