I ran the Warrior Dash early last year. Cakewalk compared to Tough Mudder.
Finished Tough Mudder in October. TM is definitely tough. Obstacles were no problem for me, but I didn't take the distance as serious as I should have.
Last October I ran the Urbanathalon in Chicago, and had been training at half marathon distances. I kind of blew off the distance of the TM because of the amount of obstacles. Rationalizing that the longest distance between obstacles wouldn't be more than 1.5 miles. Next time I will not make the same mistake.
Many people say being able to run a decent 5k is enough, but I will disagree. For me, I think having a good 10 minute pace at 8-10 miles is where I would want to be.
I would highly recommend TM. It was a blast.