Meditation is learned skill in a way that "running" in your example doesn't seem to be. Ten days of practice is more than just "ten more days of meditation in my life"; it's a hundred hours of improvement. A more reasonable comparison would be to say that a meditation retreat is like going off and doing math for ten days straight. (I've actually done the latter, and it's shocking how much you can learn and improve in a week and a half if you completely dedicate yourself to something. Total immersion really is very very powerful.)