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High step-ups (14-20 inches - or maybe even 24" depending on fitness level and your height) are plyometric in some ways because, even though they do not involve jumping, the step should involve explosive movement. It almost has to. It is calisthenic, not weight-lifting. It is tough to get up there repeatedly.
It's good to do them forward and also sideways on a box. It they are too easy for 20 reps with each leg, raise the height or grab a hand weight or kettlebell to raise the intensity.
3 sets of 15-20 on each leg is good leg exercise - quads, glutes, and hammies. With exercises like this, use a 30-40-second plank as a recovery before the next set so you aren't just standing around. These step-ups will help with your box jumps too - not to mention your mountain hiking which is the important thing because fitness may be made in the gym (and with nutrition) but it is for living life vigorously rather than being a lazy book-reading and internet-surfing slob like I used to be.
Have you been able to make progress (increase the weight) on your lifts (deads, squat, bench, etc.) over the past 6-8 months? IIRC you are lifting twice per week and doing other things, such as above, the other 3-4, and have been at it for ~2 years. So you should be past the novice phase as Rippetoe would define it.