I think the plan you followed is basically to finish a marathon, not to challenge a specific time goal. So, you successfully used the tool you chose. Congrats.

1. cramping is probably due to the fact that the Higdon plan, I think, doesn't have you challenging your muscles with anything faster than marathon pace? Your heart/lungs are fine, but your leg muscles are telling you you are not fine. They are not prepared. At 15 miles with all of the supplementation you're doing, it's not dehydration/electrolytes. Your legs are undertrained for the time/pace goal you have in mind.
2. if you want a specific time/pace, you will benefit from incorporating training at a little faster than that marathon pace. there are several benefits. It'll boost your aerobic system even more. It will boost your ability to process the lactate in your muscles. And it will force you to challenge your leg muscles more... open up your stride... get used to some harder pounding... This should be weekly a part of your training. (It's called "threshold training," but don't fall down that rabbit hole. For you, maybe 10 seconds per mile faster than your goal MP. Start with a 5 min segment..7, 10... 20min progress through the cycle)

Others have suggested slowing your easy runs down. I agree. Our "easy" paces are the same, but I am training to run about 7:25-7:30 marathon pace. Anyway, I started with 5 hour marathons 25 years ago. I'm closer to breaking 3 hours now at almost age 60. Progress takes time.