Regarding your update OP. You clearly want to meet up with your old club again so just do it!
Regarding him. There’ll never be a right time, right headspace to see him again so take some control of that situation – you know you’ll see him there so you can mentally prepare yourself for that (rather than randomly bumping into him in the street and feeling flustered and caught off-guard).
I think the fear of seeing him again is worse than it actually happening. Yes, you will react internally (that thud thud thud feeling in your chest) and yes you will feel awkward when you acknowledge/have that first conversation, but honestly, it goes. It really does. Plus, once you get into your run, that will occupy your mindspace (especially if the couple of people you were friendly with still go).
There’s also the possibility that you might see him now and feel absolutely nothing towards him, or even resentment and be left wondering what you ever saw in him – happened to me once after a year of “avoidance tactics” – I just assumed that if I saw him again I’d fall for him all over again and it would re-open old wounds, that wasn’t the case. That first conversation was still awkward but not half as bad as I expected because I just … felt nothing, romantically, for him.
Are you still in contact with the couple of people you knew from the club from before? Could you get in touch with them and briefly explain you’d love to rejoin the club but need a bit of a hand hold because he’ll probably be there and it’ll be the first time you’ve seen him since you split?
Of course, if you don’t think you’re actually over HIM yet then yes maybe leave it a bit longer but if it’s just the worry/awkwardness of seeing him again then please, don’t let that stop you.