Can I stay consistent under sub 5 for the mile?

It depends. If you do smart training and improve over time towards your goal/peak race, then yes, it will be easy to stay under 5. If you have already been doing a lot of hard training for quite a while leading up to your sub-5 performance, then it is definitely possible to stay under 5 and quite likely, but it won't be easy.

Should you run the day before a meet? I would say yes. How long depends on how many miles you have been doing. If you have only bee running 20-30 miles a week, then I would only do 1.5-2 miles with a couple very short brisk pickups thrown in. Nothing too taxing. If you have been doing a lot more mileage than 20-30 then you can stand to go an extra mile or so.

All of that depends if the race is important or not. If it is a regular meet in the middle of season, I wouldn't be afraid to run 3 miles or more if it's easy for you.

I wish I had your worries

I think the hardest part is convincing yourself that you are capable of breaking 5 minutes for the mile. Once you do, you have more of an expectation of being able to run that time. At least that's my opinion.

Um, it's January. If you're not running consistently better during the spring than you did the first week of January, something's wrong...

You did not give us enough information to tell you if you will keep PRing, that is a function of training, not the fact that you JUST broke 5 minutes in the mile. For some people that isn't a mental barrier. I have never run a mile race where my finishing time was within 4 seconds of 5:00, I went straight from 5:07 to 4:55, and never looked back. If I hadn't continued to improve my fitness, I probably would have run a couple of 5:02s in dual meets, once you are under 5 you won't always magically break it.

I can only stay under 5 minutes the mile for about 5 miles. After that I just get too tired and have to stop.