I've been doing this for 9 years now, but take my advice for what you paid for it. To answer your two questions directly:
1. Yes, you should absolutely do a test day before your first race. I almost guarantee you'll find a problem that won't pop until the car is pushed hard, so having the chance to discover that and fix it before the race is golden IMO.
2. The short answer is NO, do not attempt go all out your first race. Get a feel for ENDURANCE RACING, and how the better teams run, and are organized. Ask questions, talk to other teams, listen instead of talk. You will discover the Lemons crowd is very helpful for noobs who would have the learning attitude vs. the all-knowing one. Notice and take to heart the bold wording above!
Other useful information in no particular order:
3. Don't worry about your car being fast. Worry about MAKING IT LAST. Imagine taking your DD and driving it as hard as you can for 7 hours straight. How do you think it would do? No imagine a car about 10% as well put together, and how it would do. That's Lemons!
4. The key to #3 above...make sure the car runs cool. Besides penalties, overheating parks more cars than anything else. Don't push the redline on every shift and make sure the cooling system is in good order.
5. Practice your pit stops. No you won't be worrying about saving 5 seconds in your first race, but if you plan on doing this for a while and getting better, all the time you are stopped will hurt you 10X more than any speed you think a good lap time will gain you.
6. If you don't remember or give a flying shit about anything I or others here have said, remember its an ENDURANCE RACE. More often than not, the fastest car doesn't win. The teams that stay ON THE TRACK, and subsequently OUT OF TROUBLE are the ones that finish near the top of the standings. This is accomplished by having the mind set of remembering you don't have to pass every car, in every corner on every lap. Write it down, I guarantee you that a slower car will finish ahead of you in your first race, and probably even many more races in the future.
7. Have fun! Don't take it all too serious. The pay sucks so if you leave with a huge grin after finishing in 75th place, you have actually won. I've been doing this for 9 years, with 6 different teams, and 6 different cars. I've won (once) but have actually had more fun in every race other than the winning outing if that tells you anything.
Team Super Westerfield Bros.
'93 Acura Integra