Running
If you've ever tried running alone, you know it can be difficult to get the motivation to go out and exercise. It's easy to come up with an excuse not to go -- maybe the temperature outside isn't right or the food you ate last night is disagreeing with you. Running is typically viewed as a solitary sport, but there are large groups of fitness-minded people out there.
Running clubs are exactly what they sound like: organizations for people interested in running. Each city has different rules and activities, but they're all bound together by the idea that a community is stronger than an individual.
