I think the biggest thing in training is running. Get comfortable running at least 12 miles. And do hill running. I was unfortunately having troubles with an ankle and knee, so I didn't run at all the whole month before. Obviously, I could not walk the next day. But it was worth it.
Obstacle wise, people are going to help you out. There's a ton of teamwork that goes on. The hardest one for me was probably Mt. Everest. It's not easy running fast up a steep, slick ramp when your legs are already quite wasted. Electro shock therapy? I recommend sliding under, if you're thin enough. And the Arctic Enema is actually kind of refreshing (once you're out…).
Seriously, I had so much fun! It's a blast, despite all the scratches and bruises. I got dirty and thoroughly enjoyed it. And I also challenged myself a couple other ways: (1) I like to know what to expect, to have a plan in place. I really had no idea what I was getting into. And you can't slow down at the obstacles to strategize. You just have to go. No thinking. Just do it. And trust those around you to help. Which brings me to (2) I do not like having to rely on others. You can definitely be the crazy fit person who completes a Tough Mudder alone. But I'm not that person. And I think you lose the integrity and camaraderie of the experience by doing so.
I definitely recommend Tough Mudder, if you're looking for a fun challenge. If the 10-12 miles is too overwhelming, there are several shorter races that are similar in nature…Off the top of my head Conquer the Gauntlet, Mudderella, Savage Race, Super Hero Scramble… I encourage you to push yourself. You can do the things you thought you couldn't. Train dirty. Have fun!