My advice is to do a test to determine LT HR and train based on that, max HR is an essentially meaningless number for training. A number of factors affect HR besides just your power output (hydration, fatigue, sickness, heat, etc) so keep that in mind.

On cramps, they are most likely caused by you out riding your fitness, so to speak. Your muscles weren't ready for the kind of stress you put them through and they got mad at you. This happens to a lot of people in a race (especially tri's) because they go much harder than they are used to in training (i.e. they didn't train enough) and their muscles revolt. It sounds like you might be doing that during your regular rides too. Not every ride needs to be (or should be) your max effort.