My first ever event sign up was for Tough Mudder 2015. My work put a team together of what ended up being 20 + people for some team building and fundraising. They paid entry and transport, we just had to train.
It's not a race but you do need to be able to run as some obstacles are a good few kilometres apart. And of course you need some strength. In our group we had all levels of fitness. Some put the effort in and did some training but others didn't. We had drop outs from 2 obstacles in. Don't think you can do it if you've been drinking the night before, that's all I'm saying.
The atmosphere from when you get there really gets you in the mood! I've now completed over 15 events and Tough Mudder sticks out as the best for atmosphere hands down. You do the 'pledge' at the start which is all fun and games and then you get stuck in! One of the worst, sorry hardest, obstacles is only 3 in - the Arctic Enema. You slide down a chute under an iron gate and plunge straight in to ice water. There are officials at the side screaming at you to keep moving as you then duck twice under tyres. Definitely one of TM's iconic obstacles and a wake up call of what you are about to put your body through.
Upper body strength is a must for this course. You have to pull yourself up things, over things, carry logs, carry a team mate, crawl through long heavy tubes, swing from bars - it's a lot. There is a lot of running too and jumping so make sure your knees are strong. Be prepared for injuries. We had one sprained ankle, one heavily bruised knee and one broken sinus bone (the poor women's face literally blew up when she blew her nose). My bruises lasted a good few weeks.
I would highly recommend anyone to do Tough Mudder but I would say the following - do it in a small group, 20 was ridiculous
- train by running and upper body strength, if you're not prepared to put the work in don't do the event
- buy your ticket early, although my work paid for mine I know the prices are extremely steep!
- be prepared for an all day event
- be prepared to see all kinds of sights! Fancy dress is optional although it seems it's the norm!

I'm doing it again this year with my brother this time so I think that speaks volumes!