Answer:
keep running. the more you run, the more you build your stamina up. If you're in school, cross country is a good thing to do. It helped me. If you already do cross country, or you don't have the resources to get on a team, make a running schedule. Run two miles a day. Or run 2 miles a week, or five miles a week, or three miles a week. but don't run too much, you could hurt yourself. The key to long distance is to keep running. And don't stop. Keep your sched.