I've been doing some internet research, and a lot of places say to become truly conditioned/fit is to do high intensity workouts for short periods of time. This is supposed to get you more in shape than any long distance, low energy run.
I've been looking into buying one of these products:
http://www.rosstraining.com/products.html
Apparently it's very good and he uses it to train athletes across the world. I was wondering if I had several months to train for the 18x program, if I should start busting my ass with Ross Enamait's stuff, or start Ruck marching with lots of sit-ups and push-ups.
Thanks guys.