I'm very turning 38 this year and active and lift weights regularly, both upper and lower body, and I remember after my first pool session this past November, my hamstrings definitely were aching a little bit. Not in the sense that it hurt but in the sense that it felt a little bit like the days after I do squats and leg presses at the gym. But I was in the pool and kicking for 4 hours, in positions I'm not used to being, so I'd say it is pretty normal for the course.
Then when I did my checkout dives for my OW and AOW, I noticed my hamstrings didn't really ache like before, however they felt like I had worked them for sure and when I was doing large kicks I could feel it a bit since I'm not used to stretching my legs that far.
I do remember though everyone saying how exhausted they are but after 3 days of 3 dives each day, I felt great and could have kept going on if I wasn't there with my wife and kid. I slept like a baby those nights though lol.
I will say though my first attempt at my OW (not the time mentioned up above) I did the first 3 dives and was very achy after that day, but I was also sick and that is why I didn't complete my OW that time, because I couldn't do the last dive. I chalked up the achyness to just being sick.
If your mask is constantly flooding, you should look into getting a new mask, because that typically means the fit isn't good for you.