i usually take 4 or 5 days off after the cramps, it used to happen a lot in the start but now it's been 7 months since iv'e been doing it so it doesn't happen anymore....last time it happened when i took a 4 hr long walk 2 months ago.
Two reasons: Lack of circulation, which can be corrected by adequate stretching, rotating of the calf and massaging the areas around the hip joint, at the back of the knee and at the ball of the foot that carry large conduits for peripheral circulation. Also, get yourself checked for arteriosclerosis.
A lack of essential vitamins and minerals: especially low potassium. Eat lotsa bananas, and see how you feel.
Stretching before running is a HUGE misconception! Do NOT stretch before running! If you really want to warm up and prevent cramps, walk, or jog lightly for 10-15 minutes. Then you can stretch or just sprint if you want. I was also surprised when my personal trainer told me not to ever stretch before running. This must be done AFTER your runup (dont wait too long or you will risk injury). Also avoid ballistic stretching, infact look it up over the internet, as to the techniques of doing it properly and take printouts to the gym. Eventually you'll get used to it and memorize the technique, you wont need to carry them with you. There is also a 'cage' in various gyms/athletic center where you can stretch different sets of muscles very conveniently.
If you're regular, do it daily you will eventually get to a point where you wont get muscle cramps.