I have an opinion, which I will entirely fail to support by offering any facts, that one's activity level BEFORE age 50 may have as much to do with fitness as your activity level AFTER age 50. If you are already exercising 420 min/week, it'll be easier not to drop to a poor fitness level if/when age-related metabolic changes kick in.
By the way a bunch of people around here are doing one mile a day Thanksgiving through Christmas. It was a suggestion in Runner's World, the idea being that if you go out for a mile, you're likely to go further, and even if you don't, it's still a mile. Depending on their fitness level some of them are doing a mile walk, or 10 min walk, or 10 min run, etc. But it's daily, whatever you choose.
2009 Trek 7.2FX WSD, brooks Champion Flyer S, commuter bike