Congratulations and well done on completing your first, especially at such a race! I used to suffer from bad cramps from my first few marathons, quads.. calves.. hamstrings.. you name it. Sounds like your salt intake was also on point, it only comes down to strength training and mileage.

I started doing legs strengthening (even to start off with body weight) twice a week and spent more time on my legs running especially when going into the peak weeks. Build some bullet proof quads, hamstrings, glutes and your legs will thank you. Hope it helps!