Many runners enjoy running on trails, and trail running can provide relief from the road's asphalt or cement. But running the same distance on a trail will take longer than it will on the road, and you'll be working harder to maintain your footing. Some trail running requires a combination of running and hiking depending on the difficulty of the terrain. If trail running appeals to you, it won't hurt your preparation for a marathon, but be prepared to walk or climb over rough spots and allow more time to complete the run. A 6-mile trail run may leave you more tired than a 10-mile run on the road.
You can run on most trails in any running shoes without a problem. But if you plan to do a lot of trail running, especially on uneven or hilly trails, consider investing in a pair of trail running shoes for the additional traction that these shoes provide. But wear your standard training shoes for road runs. Running on the road will wear out the tread of your trail shoes and destroy the benefit of that extra traction.