Does Spartan Race give you any ideas or tools that you can use to market these classes? Can you advertise your classes through their site? I would try to contact your local running groups, running stores like Fleet Feet (because I know Spartan Race likes to promote their events at these stores), outdoor stores (like Dick’s, Sports Authority if there is one close to you, etc.) and maybe create a meet up group (meetup.com). I hope this helps.
Who is the target group for your classes? Then find out where they hang out and market to that area. I often search for facebook groups in my area that target my niche group. Or work with other businesses, social groups, etc that include my target niche. You can also think of where these people go to hang out (a bar, coffee shop, etc) and see if you can offer a trial free class at that location to gain interest. It doesn’t have to be a full class, a 10-20 minute teaser works great too.
As everyone already stated, the first step is locating the prospective clients. Once you figure out where to connect with that particular group of people, focus on either an online presence (Facebook, Meetup, etc.) or old school handouts (similar outdoor events, parks, etc.). There is no “easy” answer, just a little homework…