I started out with walking every day for about an hour. Then running for 20 minutes with stretching and pushups (I think it was 50?) before and after, six days a week. I had problems with my knees for the first time in my life, and then it moved to my arches. I tried different shoes, and some helped more than others. I really DO NOT enjoy running anyway, so it was a slog every single day. Then I got an exercise bike that worked with the arms as well. A couple days running and four days riding for 20-30 minutes. I eventually gave up the running unless I was away from my bike. I went, I think, six years without deviating from my routine, no matter where I was or how I was feeling. I then moved to the bike six days a week, eventually dropping that down to five days a week. That's where I'm at now, and I'm far less strict. I don't often break from my schedule, but I can be coerced.
I've been athletic all my life, but I don't know anything about fitness, other than my own. The big rule I kept in mind was getting your heart rate up for 20+ minutes straight. Sweating isn't enough. Working hard in the yard isn't enough. From what I understand, the key for older folk is getting that heart rate up and keeping it there for 20+ minutes. Strength training is also important to keep your muscle mass and for balance. My bike that works both arms and legs has served me surprisingly well for my upper body. I used free weights when I was younger, and I hate it almost as much as I do running.
If I was changing things up, I'd move to jumping rope, something like bicycling outdoors and inside on rollers, or a rowing machine. If I enjoyed swimming, that would also be a consideration. Jumping rope would be the first one, though. Older folk need to challenge their balance, and jumping rope works on balance, agility, quickness, and timing, and if you've ever jumped rope, you know it is a crazy good cardio exercise. Jumping rope will humble you for a couple months, until you get in shape. Great for your feet and ankles, too. If I had money to burn, I'd get a good rowing machine. They're great for every muscle group, especially your core. I do have a Nordic Track I picked up for peanuts (expensive new, but about the cost of a meal on the used market), but I haven't used it since tuning it up. No reason. Nordic Track/cross-country skiing is a great exercise physically and cardio wise. I really should be alternating the exercise bike and the Nordic Track. They use so dang much space, and I'm too lazy to keep moving it back and forth to use it.
. Talking about heart rate!