I'm a college swimmer, have swam competitively all of my life. My suggestions for doing well on a 500 yd swim are this:
1. You should practice longer or the same distances than your race will be. Therefore, if you're swimming a 500, your practice should be on things that are at least 500 long, most of the time. Not to say you can't do some shorter stuff to mix it up.
2. Flip turns: The "T" is always the same distance from the wall at every pool, and the solid color on the lane lines should be the same distance at every pool (although this often changes as lane lines get beaten up). To find the proper distance from the wall where you should start your turn, do the following.
Place your feet flat on the wall and strech your body out down the lane facing away from the wall. Point your arms straight down to the bottom and note where they are pointing, i.e. how far from the T they are. This is the point at which you should start your flip for the turn. Whatever stroke takes you past that mark should be the stroke that you start flipping on. Always flip straight over and hit the wall on your back. You do not roll over on to your stomach until you leave the wall, this is the fastest and most efficient way to do a turn. When you come off the wall, do some dolphin kick or flutter kick until you are at least outside of the solid lane line. NEVER breathe on your first stroke, it breaks your rhythm and slows the entire length down. You should be able to turn on either your left or right arm to avoid the need to take short strokes to turn on your strongest arm.
3. A workout should generally consists of 4 parts. A warm-up, generally long, probably about 800-1000 yards. Next some stroke/drill work, about 500 yards. After that, some "set" work. This is hard and fast swimming where you really get your workout, about 500-1000 yards. End with about 200-500 yards of cooldown. This gives you about a 2500-3000 yard set workout. For someone who's swam his entire life (i.e. me) this take about 1-1.5 hours depending on what I do and how hard I take it. 2000 yards can take me as little as 30 minutes. A good general guideline is that a sprint 100 should take about 1 minute to 1:10. When you're doing longer work, like 500+, a 100 can take as long as 1:30-1:40 in practice. A good 500 time in competitive swimming is about 6 minutes (not in college, you basically have to be below 5:30 to be competitive outside of D1, which is insane-fast). Anyway, that's my suggestions about swimming. Any questions, feel free to post.