Pamestique 

It could be anything but most likely poor hydration. Do you take BP meds, like a diuretic? That can cause loss of electrolites (salt) and lead to cramping. Staying hydrated is a relative issue. After a 40 mile ride just how much fluid have you taken in? In summer heat it should be at least 70 - 100 fl ounces. That's 2 - 3 large bottles. There are some good supplements out there like Motor Tabs etc. Also is it possible you are over training, riding too much not allowing the legs and muscles to get some rest? That will also cause cramping.

And lastly, you know the old joke "Doc, it only hurts when I do this..." "Well don't do that anymore!" If standing causes the cramp, don't stand. Lower the gearing and spin up taking the rest and relief offered.

Also wildwood bought up an excellent point... are you eating? I learned after suffering heat exhaustion and being hospitalized that in order to retain fluids one must take in food. The food keeps the fluids in the system otherwise it sweats out. Every 20 - 30 miles you should be taken in some protein or carbs. Next time at 30 miles, try a Cliff or Balance bar, some nuts or dried fruit, fig bars, whatever, drink some electrolyte fluid and see if that doesn't help.