Beginning Fitness

Hi. I am looking to do Mt. St. Helens at the end of September and need to start training ASAP! I want to follow the 6 week beginner plan but have questions.

For the step ups/down/sides, do you just use a regular step, or would you use a park bench or something of that height? Or maybe a step class plank with risers? Just wondering how high the step needs to be.

And for the Aerobic section there is a stairs workout at 70% MHR. Can you use a stair machine at the gym (you know, it looks light a rolling flight of stairs that you walk on).

Thanks for any insight...