I know that feeling. The day of is pretty good. The day after is hellish. Sadly, I've found that minima,l soft exercise works for me. I used to be a quite active 72 year old. Not so much now. It's finding a balance I think, because when we feel good, we tend to overdo it and the next day is not good. Just keep moving, not so strenuously. Maybe float rather than front crawl, which I tried a few weeks ago. Enjoy the swim though.