The expert racers are probably running tubeless, but the probably also have a tightly rolled up tube or two in their jersey pocket..
Spare tubes, a CO2 cannister, a multitool, a spare connex type link for your chain. Tube & multitool in your pockets (jersey or camelback), CO2 taped to your seatpost, chain link taped whereever. energy packets stuffed into your shorts leg or taped to your handlebars. A tire iron or two (but you could use your quick release handle for that).
Get one of those key boxes & leave your keys - there is NOTHING like having to retrace an entire race course to look for tham after they inadvertantly fly out of your pocket! Talk someone in to going to the race with you for support.
As with mud, life, too, slides by.