Be confident in the work you did to prepare for the race. Take a look back at your training logs to remind yourself that you've done everything possible to prepare. The race is the fun part where you get to see the hard work pay off. Enjoy it.
Like physical work, if you don't practice, you shouldn't expect results.
Stepping outside the comfort zone is the price I pay to find out how good I can be. If I planned on backing off every time running got difficult I would hang up my shoes and take up knitting.