I put together some of my favorite pre-season Mtn biking exercises for you! 
Warm up your legs with these floor exercises 3 sets of 20 reps per leg. 
Clamshells banded. The blue band adds medium resistance. I recommend the band for added resistance and to help build more strength. 
Hipe Raises 
Start your exercises with both feet down. 20 reps full range then hold at the top and perform 20 reps w quick glute squeezes. Perform all 3 sets
Then perform the exact same sequence but with a single leg.

Drinking Birds with a dumbell or a kettlebell perform all on one side 20 reps then go to other side 3sets. Hips and shoulders are parallel to the ground.

Banded goblet squats. 20 Reps 3 sets
May be performed holding a dumbbell or kettlebell or no weight at all.

Bulgarian split squat with back foot elevated and hands overhead. 15 reps on 1 leg then 15 on other leg 3 sets
May be performed by holding a dumbell or kettlebells in one hand at center of chest or 2 by your sides.

Walking lunges 20 – 30 steps 3 sets. Holding 2 kettlebells or dumbbells for added extra resistance to build strength and stamina.
Single-arm kettlebell swings 15 each hand or 20 both hands 3sets. May also be performed by alternating each hand with precise form and skill or double hand swings. Pick your poison :} working endurance. Ensure you’re using your glutes and hips not your low back.