The most important factor in maintaining fitness for life seems to be how long one can maintain the exercise activity. This is a major reason why I am a transportation cyclist. I don't own a car, am too cheap to use ride-sharing frequently, and dislike buses, so I have no choice but to ride almost everywhere.
No excuses are valid. Don't feel like it? Doesn't matter. I don't want to ride most mornings. Cold weather or rain? Too bad, no choice still. Sick? Take a day off for your health.
Several people I have met seem to think riding as often as I do requires superhuman willpower. But I am no more successful at maintaining other exercise in my life. I used to run more regularly, but I haven't had the time for a while. So I understand how difficult maintaining an exercise habit is. That's why I leave nothing to chance.
If you can organize your day such that exercise is unavoidable, do it.
(And it doesn't need to be cycling. Walking to work is good too, though much more time consuming and lower intensity. I also think doing exercise in a group with social costs for skipping out is good, though not as unavoidable as transportation cycling.)