Possibly intense physical and mental challenges due to the duration in the backcountry.
A Level 5 trip demands excellent physical and psychological condition. Just going to the gym 3 days per week probably won’t cut it.
Depending on your base level of fitness, you should be exercising 5 – 6 days per week, with at least one long run, hike or bike on one of those days. By “long” we mean at least several hours. Anything that gets your legs moving and your heart pumping is good. Add “psychological” challenges once in a while; the extra half mile sprint at the end of a long run, a long hill climb, etc.