Like one of the other posters, I had to learn that the myth of 'a little bit every day' doesn't work for me. I've been doing marked steps up in functionality very slowly by every January setting a physically challenging daily goal, and at the end of January (whether I met it at all, sometime, every day) stopping that. For me, this is easy to manage, because it fits with school holidays, and so when school goes back, I stop whatever the thing is.

As a result, I've gone from unable to run at all, to being able to do a couple hundred metres at a slow run.

I've also had to move outside the standard kinds of get fit activity that are common around here (gym, running/jogging, swimming) and find something that makes me work hard without making breathing so hard that I don't bother. As a result, I've taken up figure skating (hello mid-life crisis bad ideas) and ballet, both of which are doing great things for muscle strength and my cardio-vascular health.