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.