I've run two Tough Mudders now and can safely say that if I'd stuck to Primal Blueprint Fitness, I would have been able to do a smaller race, like Warrior Dash, with no problem. The main thing is to make sure you are getting some type of interval on your moving slow days (hills, trails, etc.) and really sticking to the strength training and sprinting part.
If you aim to do a Tough Mudder or something else in the 10-12 mile range with a bit tougher obstacles, AND the goal for you is to get through the race plus the obstacles, then I feel you need to up the ante in training. I started Crossfit 2-3x a week for my first Tough Mudder and handled the running just fine even without never having run that distance before. The second one, I bumped Crossfit up to 4-5x a week and ran it faster than another guy that was a marathoner (oh, yeah, and he couldn't pull himself up over the 6 ft. walls ).
Not saying if you want to do a longer race you gotta do Crossfit, but definitely HIIT with some significant compound movement strength training would help. For the 5k length races, PBF should be fine, especially since crowds are usually big and you get some decent rest time. Good luck!
~All luck is earned in the end.~