What sport or exercises are the best if you want to run for a long time without having cramps?

None in particular: If you plan on running for a long period of time, the type of exercise does not matter in my opinion. Rather, the key is adequate stretching (though the need for stretching is a highly debated topic, though I am a proponent), and adequate hydration. Bananas are a good source of potassium, and can help fend off cramps, as can good old gatorade.

Running: To run a long time , you have to train . That type of training involves long distance running as well as interval training. Adequate hydration, proper foods etc are all part of that equation. No one runs a marathon by stretching and then just doing it. The training is arduous and daunting - but worth it of that is your goal.