100% Best thing you can do is spend time with a pack in uneven terrain. As much as you can handle. The more easy volume the better getting progressively longer and longer each week as you approach the season but also making sure every 4th week or so you take an easier week to recover. (3 on, 1 easier but not totally off)
I've been mountain biking, training and racing, pretty seriously over the last 3-4 years Never been in better cardio and endurance shape. Probably never been in worse mountain hunting shape since I started doing it.
I'm trying to get that fixed for this fall but mostly comes down to basic gym workouts (Total Body/Core plus legs with a focus on Squats, Deadlifts, and lower leg / knee rehab type stuff) and as much walking with a pack that I can find time for in addition to time on the bike.