Of course you can market Taekwon-Do, or any martial art, for a high level of fitness. You might even talk to a gym or other fitness facility about hosting a non-"cardio kickboxing" class that is geared to people who are interested in increasing their overall fitness level. It could take a bit of time to build up a student base but it's possible.
I have to say, however, that comparing things to "Tough Mudder" is an analogy that limps a bit. TM seems to me to be more analogous to a shugyo type of training. That it, it's an experience of hardcore training that is geared to people who already have a good base in training. From what little I have seen of it, TM is billed as a special event, one that you don't do everyday. Shugyo is an intense training experience that is undertaken by people who already train and is even more intense than their normal training regime. It's supposed to really put you through your paces, just like Tough Mudder. As such I don't know if having TM as an analogy for general MA training is accurate.
Pax,
Chris