What are the pros and cons of running on trails compared with roads?
Running mostly on trails is good for reducing impact on your muscles, joints and bones, which reduces the risk of injury, as long as you avoid very uneven surfaces. The other thing is to ensure there’s not too much loose gravel; frequent slight slips on toe-off can cause problems, particularly with your Achilles’ tendons.
However, unless you’re running on extremely good trails you could miss out on the pace that’s needed in some components of training. Also, if you will be racing on the road, a lack of adaptation to that surface could increase your chances of impact fatigue during the event. You could be more likely to pick up an injury if you suddenly run your hardest on an unfamiliar surface.