Right now, your weekly mileage is 30. Increase that by 10% per week, not by making one ride longer, but by doing more shorter rides. When you get up to 50 miles a week, one 30 mile ride will be easy.
Muscle cramps are caused by working muscles much harder than they are used to. There can also be a hydration/electrolyte component, though in 30 miles those would be insignificant.
You can break a muscle cramp with pickle juice. Small bottles, easy to carry, are usually available in bike shops now.
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