Congratulations on your acceptance. As you know, most of the coursework is drawn from the first year medical student curriculum, so the difficulty closely approximates the medical school experience. Your MS-1 peers do have a bunch of extra obligations in the way of clinical coursework, gross lab dissections, etc. so you should have extra time to study as an SMP student. I'm sure you've heard this before but the "difficult" part of med school comes down to being diligent and keeping up with the rapid pace of lecture. Beyond that it's not much more challenging than UG-level classes (maybe not at all).
I agree with your "one shot" comments. This program can help A LOT in getting in to med school but you need to commit to it fully.