Thanks GeeeM. Yes, perhaps a short email to past competitors might be a good idea. Thanks for that!
We have a section on our website for volunteers explaining duties, benefits etc. so already have that base covered.
Indeed, perhaps there is an opportunity for someone to create a volunteer database, so that people looking to get involved can get involved. I've volunteered at quite a few races organised by friends or my running club, helping out at aid stations and sweeping, and I've found it incredibly rewarding putting something back into a sport I love. Perhaps there are more people out there who would be keen to get involved, but just don't know about the opportunities available to them.