Depends on what you are running for…if you are training for a Marathon, then, try to train on a similar terrain. For instance, it is Quite Different running the Marine Corp Marathon in Washington DC when compared with the Boston Marathon.
If you are running to stay fit, then, running on trails will enable you to run on uneven surfaces too…as with any kind of training, you need to take it easy at first and Progressively Increase Difficulty and Workload.
The elevation and slope play a factor too, of course.
Generally speaking, the hardest surface to run on is Concrete. Try to avoid running on Concrete. Tar Roads are better than Concrete Surfaces to run on. Running on grass is perhaps one of the best. Trails will definitely be uneven…hardness will depend on the specific location.
Other IMPORTANT things to take note are…
Do be careful and prepared about potentially running into animals, depending upon what animals are present, at that location…Deer, Snakes, Bear, Mountain Lion… And, ensure you know your way in and out. Let your Family, Friends know your Running Location and Schedule, so they can keep a track. Running in a Group is way more Safe than Running Alone, especially on Forest Trails.
Safety First and Above anything else.
Hope this helps!