I have been working for over a month at "getting into shape". I'm mid-60s, 6'4", probably less now, and 223 pounds, some of that muscle, some is a paunched gut. I USED to be able to get into decent shape in 3-4-5 days. No longer. Do NOT let yourself sit behind a desk for two decades and think you can snap back into shape with relative ease.
Part of the problem is twinged muscles at times. I'm stretching and warming and all that, but one wrong step and I'll get a twitch in the quad that can turn into something worse. My heart rate at rest is down to 50 now, from about 72 or so, so that is great, but running remains tough.
So, a word to the wise, stay in decent shape.