Lately I've been slacking a bit regarding my workouts. When the mileage increases, the less strength and cross training I want to do. But Wednesday is no excuses day and I hit the dungeon.
First the treadmill, for five excruciatingly boring miles. Then most of my strength stuff is body weight, most of which is focused on the "core".
For road running, form is everything, and you won't last long if your core is weak. For the trails, especially the more technical ones, it's more about survival and being able to twist and move... both to get your feet where they are supposed to be, and to shift your weight to stay balanced.
Ultimately that's what I tell myself, that it's for my own protection. Gotta make it through that torture somehow.