I would hesitate to answer that yes, if you get gut-wrenching anxiety for days, then perhaps you should take smaller steps before that.
But how do you do that, is the question. You say the other class doesn't cause you any anxiety at all? Or does it cause a little? What is the step to take to get you to the running club?
First off, I think you need to practice the rational strategies about this running club. Anticipatory anxiety - what do we do? Use ANTs stoppage techniques, slow down, use the conditional, turn the tables. Don't dwell on the thought. Find distractions. Keep practicing this each time it comes up. Gut wrenching anticipatory anxiety for three days - I can understand this, but I also must ask if you are reminding yourself of the rational? Are you using the cognitive therapy to get you closer to being ready for this club, or are you forgetting to use your strategies for this upcoming event?
How might you break this down? It's a running club. If you feel that is more serious, running with people, expectations, can you ask one person to join you for a run? You don't really need an excuse. Maybe you could have a reason, that you want to test something in your run and having a partner would be a good help. Maybe with someone you feel okay with, a simple invitation to job is enough. But treat this as help and practice for the next step. You can ask the person to help you set up the situation as it would be in the running club.
Sometimes I don't think there are necessarily ways to break activities into smaller steps. I think it's the cognitive preparation, reminding oneself of the rational, reminding oneself to slow down and focus out. There may also be ways to break something down into smaller chunks, yes, but even there, we must be cognitively going about it in the right way so that the event becomes constructive for our changing thoughts and feelings.