Terrain as Strength Training
Look at the above photo. That ice has been shaped by occasional cars driving on it, the occasion SKORA shoe, wind, rain, above freezing temperatures, and below freezing temperatures. It is rough, absurdly uneven, and crinkles like stepping on dried leaves.
So, why would I choose to run on that instead of the road?
For a number of reasons. Mainly, running on that uneven surface is making me a stronger runner. Running primarily on flat pavement is extremely repetitive and does not stress the body in multiple ways. But, running on that ice, every footstrike is different than the 100 previous and the 100 that are to follow. Each step, stressing my body in a slightly different way, giving my legs slightly different stimulation. Making it stronger!
Now, speed will slow and effort will go up while running on ice or snow. But my coach recently reminded me that my body does not care what the pace was at the end of a run, only what the effort level was.
Trail running works the same way. You will likely have a much slower overall pace on trails, but that is simply because your body must work harder on hills and technical terrain. Just know that you are stronger because of it!
If you are a primarily pavement runner, take the transition to ice or trails carefully. This new stress on your (primarily) ankles will make your body stronger in the long run, however too much over-stress in a short time could cause injury. As with any change in running, be in new shoes, new speed-work new running surface, or new distance, gradually move into it.