What you do all summer shows up in September whether you invited it or not.

A long unstructured stretch does not pause the accumulation, it just hides it. Whatever you did repeatedly across those weeks has been building something, in the direction of fitness or rust, and the return to structure is simply when it becomes visible. Nothing is being punished; it is only arithmetic catching up.

Pick one thing you would like September to find already in place. Do a small version of it a few times a week, and let the rest of the time be genuinely off.