At 69, I have to also agree that you first talk to your doctor. Assuming an OK there, I am really a big advocate of simply walking, either treadmill or outside. I use both. The treadmill gives me consistency in angle, speed etc to measure progress. Outside walking (for me) seems to work the muscles slightly differently than the treadmill, which helps. Also, I find walking outside cheers me up.

Whatever you choose, IMO the most important thing is consistency and patience. Start slowly and as it becomes routine and easy, slowly increase the effort. Positive results will come slowly, but they will come.