The best surface to run on is one that's ?

1. Grass
At its best, the grassland of parks, golf courses and football pitches provides the purest, most natural surface for running. Areas where sheep graze are often home to fine, close-cropped turf, too.
Pros: While grass is soft and easy on the legs in terms of impact, it actually makes your muscles work hard. This builds strength and means you’ll notice the difference when you return to the road. When it’s flat, it provides an excellent speedwork surface (spikes may be necessary in wetter conditions) and, unlike a track, can give you space to run whole repetitions without having to make tight turns.
Cons: Most grassland is uneven and can be dangerous for runners with unstable ankles. It can also be slippery when wet, runners with allergies may suffer more symptoms when running on it, and its softness can tire legs surprisingly quickly. Finally, of course, while the very best grass for running is often found on bowling greens and golf courses, the owners are not always happy to discover runners on their hallowed turf.
Conclusion: If you can find a flat, even stretch of it, grass is the best training surface for most runners, especially as you get older.