Just did one this past weekend. If anyone is interested in it, DO IT! It hurt, yes. It sucked, yes. It was a PITA to train for, yes. But finishing was one of the most sublime experiences ever!
My workout for the past 6 months was basically kettlebell clean and presses and swings 3 x week, bodyweight exercises (the Convict Conditioning program) of pull-ups, push-ups, leg raises, and squats, and lots and lots of running (I was up to 3-5 miles twice a week and a 8-10 miler on the weekends).
I finished in 4 hours, 3 minutes, which put me right at the top 30% for all men and 25% for men my age (36-39). Pretty nice validation of my training program, made all those stinking hot and sweaty summer work-outs worth it :P
Training FAILURES: Cramps were my biggest limiter. After mile 6 (of 12-14) calf cramps were frequent, usually happening right as I jumped on a wall or was -almost- to the top of a mud slick hill :P Need way more hydration and electrolyte replacement.
Need to do WAY MORE hill sprints and lower body work. Carrying weights was easy. Pulling myself over walls and up ropes was easy. Climbing YET ANOTHER DAMNED HILL was excruciating. I have a nature trail near my house that is basically straight up, then straight down. I thought it was too steep to be good practice and instead spent my running time at the paved, mostly flat greenway. WRONG! That nature trail will be my new friend for the next training cycle. Interval sprints, ruck marches, anything that overloads and develops the legs. They can not be strong enough. Running for distance is cool, but there are so many times where you simply can't run (trail too steep, too narrow, too crowded, underwater) that max run distance is secondary to peak effort and VO2max.
Run wet and muddy. Find shoes that can be easily removed and washed out. My New Balance 993s were great for the first 5 miles, but eventually became completely waterlogged and sand filled. The laces wouldn't unknot and trying to wedge my foot back inside just caused lots of cramping. Definitely going to invest in a minimalist shoe or 5 toed shoe, something that won't hold water and can be more easily removed if necessary (downside is potential loss of said show in a mud pit, do they make 5 toed ghille broughes that lace to the knee?
Finally, RUN WITH A FRIEND! We made lots of temporary trail buddies, but it was my longtime friend who pulled me through it, made me run when I wanted to walk, and motivated me during this past summer of training. I'm sure his desire to not fall behind me did the same for him
Get someone, ANYONE, to do this with you.