When running a road race, power is easy to understand and use. Based on your critical power number, you can select your power target for the entire race and stick to that one number (if the race does not have steep inclines/declines).
On trail, if you run in technical terrain and face steep terrain, you can no longer use a single power number. You can also expect that your target power numbers will change after the initial 6 hours of a long race. The best resource to understand how to use power in long events in technical terrain is the following webinar by Andy DuBois: