Back injury from overdoing it?

I'm grateful to be able to reach out with a question. So im new to swimming. Never been good at it so I got some long fins to help my kick. Long story short I got so into it having so much fun I would do like 30 round trip laps every day on the kick board with no stretching afterwards. In the beginning I felt just fine. But after around two months I realized I was starting to get super tight all up and down my legs. So much so that my hamstrings are now like steel and pulling very tightly on my low back. Its clear that i probably need to cut down how much I swim and also stretch as much as I can each time I finish. but the question is how to proceed now that I have steel cables for muscles...I feel like I need to make up for two months of no stretching but that is obviously impossible. Does anyone have experience doing something foolish like this due to over ambition and just plain enjoying themselves? I'm bummed because my back has been aching even after a week of rest and I want to get back into it. Any thoughts? im 44 if it makes any difference. Thanks and glad to be here.