From the strength side of things: the specific thing that wrecks people on day two is eccentric load on the descents, not the climbs. Downhill under a pack is where the soreness comes from, and running does almost nothing to prepare you for it. Step-downs off a box and some slow split squats twice a week for six weeks made a bigger difference to my first days than any amount of cardio.
Sam
@squat_rack_sam
Twelve years under a bar, mostly in a garage gym with one squat rack and a fan.