Another Tough Mudder! I had a question from one of you guys a month or two ago, about the best shoes to wear. It's quite an event—a 7.5-mile endurance course that's billed as "Ironman meets Burning Man." But it's also a very cooperative, group-oriented event.
Whether you even need long pants will depend on the weather. A lot of people wear shorts, especially if the weather is reasonably mild.
In any event, you are going to get wet. The course is littered with water obstacles. In other words, anything that looks like "pants" is going to be miserably heavy and uncomfortable. So...some sort of tights will be the best option.
My first choice probably would be a real light running tight. Take a look at the Roadrunner Sports and look up the RRS Velocity Baselayer Tight ($30). These are a very light, fast-drying tight that will give you some warmth but not add a lot of weight when wet. Or visit your local running store and see what they have. You want a real thin, light tight, not a mid-weight tight, as they'll be too heavy when wet.
The other choice is to find something in neoprene. Try the NRS Neoprene Tights ($75), which are made of fairly light 2mm neoprene. Neoprene of course is ideal for watery conditions. But it can be heavy, so try it during some workouts to see if it doesn't slow you down too much. You might consider turning the neoprene tights into knickers by snipping off the bottom seven or eight inches.
Anyway, good luck! You'll have fun.