What heights are people using for the step ups in the training plans?
For me, it varies on where I'm training, be it at home or at work(have a gym at work). At home, I do step ups off my deck which is about 12-18" off the ground. At work, we have a plyo box that has a height of 24" if memory serves me correctly.
In the wilderness your step will vary depending on terrain and your hight. It's ok to switch it up, the important thing is to get outside and challenge yourself on a consistent basis.