For me routine works, obviously you don’t have a chance to build one in one event.
So you could try a checklist/plan so you can stop thinking about it. Decide what you need to do before the race and then write it down. That way you won’t stress that you may have forgotten something.
Obsessive organization helps too. I always run through kit, equipment, clothes and food the night before a race. Have a contingency. I prepare for the forecast but think, what will I do if it changes and becomes wet/hot etc. Again, often not necessary but I find doing something is better than nothing when it comes to nerves.
I’d do a few easy sessions. Before bike races I’ll normally go out on a club ride the day before. Company and riding is distracting and it makes my legs feel better to have an easy spin out. Club runs are ideal as the pace is limited by other people but there is still the odd opportunity for a short hard effort at the café sprint to loosen the legs.