why do you get a cramp after walking so long?

Answer 1

Could be anything -

Not used to exercise.
Not drinking enough water to flush away the lactic acid that accummulates with activity.
Your muscles hate you.
Not eating properly...

Examine what you are doing on the whole. Is your diet healthy? Do you stretch before walking? Are you generally walking after long periods of sitting? You'll need to give out more info before one answer will make sense.

Answer 2

Your body is sweating out the salts it needs. Just like with distance runners you should be drinking a sports drink to replace what your body needs.

Answer 3

mostly because you legs aren't exposed to that much exercise at one time.