Careful on beaches. Sand is more treacherous than you'd think, and you'd be surprised at how many college athletes leave for spring break healthy, do their training on the beach, and come back with plantar fasciitis or achilles tendonitis. The softer the surface, the harder it can potentially be on tendons. However, harder surfaces mean greater impact (tougher on joints and a greater possibility of stress fractures), so it's all about striking a balance based on what your body can handle. I agree that concrete is terrible, but asphalt roads aren't nearly as bad as concrete. Tracks, on the other hand, are useless for substantial mileage. Other than being boring, many of them also have rules in place that force you to only run in the right direction, and always running the same way around a track can precipitate ITBS and other issues in your inside leg. If you can find a track that lets you change direction, this is less than a problem. I save the track solely for speedwork. The rest of my mileage is done either on roads or the dirt shoulder on the side of the road.