Race Questions for newbie

yes. 

what was the other question?

You sure that wasn't for 400 yrds?  That would be the 1/4 of your race and match the distance convention you listed the other 2 sports in.

Anyway, your times are very close to mine.  My 5k PR was a 20:45, my swim time for the 500 is around 12', or 2:30 /100 and my bike is a little faster at 22mph over a 20k TT.

Anyway, you will be fine to finish and should do rather well.  You can read all the advice about pacing that you want, but it won't mean a dang thing on race day.  The only way to know how to pace yourself, is to run the race and see how you feel.  When I did my race, I tried to pace.  I paced well through the swim, and hammered on the bike, knowing I was pushing it a little, but really no harder then my hardest brick.  In the end, the run sucked more then usual, as I never did the swim before the bike and didn't do either in race conditions.  So chances are you will not run this race perfectly.  You will either push too hard or not hard enough and you will know exactly which you did when you cross the line.  Take note of that and adjust your training/pacing plan for the next race.

you should also have an advantage with your marathon training.  Even in the sprint form, tris are very demanding of endurance systems.   While a sprint running race may last 20 min, a sprint tri lasts at least 30 min, usually up to an hour, depending on length.  That's a long time to be going all out, so having the higher endurance and the better developed core muscles that are used to that kind of abuse will help out tremendously.

Good luck and let us know how you do