Regardless of the event you’re preparing for you should aim to mix up the surfaces you’re running on each week. Mixing up the surfaces between pavement, dirt, and grass can help strengthen all the little proprioception muscles that help you balance and stabilize while you’re running.
Varying the surfaces regularly changes the way your foot lands ever so slightly, and the way the impact forces are distributed throughout your body. That variety will help build a more well-rounded, stronger runner that is more resistant to injury. Even if you’re planning on racing only on roads that variety will help. When training for a road marathon you want to get 50-60% of your running in on pavement, but you should still make sure you’re mixing other softer surfaces in on a regular basis.