Good story.
You eventually learn where the line is, then when you pass someone approaching a turn for the first time, you wind up driving deep into the turn and your approach is now off-line because the car you are passing is on your "usual" line. It's hard to judge your speed entering this way and you can run off on corner exit. The effects of nearby cars on your concentration need to be considered in advance since they are pretty significant. Especially when you are new at it. I spent half a race in June at Sears Pt with a faster car behind me. I was just a bit slower in the turns but had more power than he did. If he could have gotten a fender up beside me, I would have let him go ahead but he couldn't do it, so I just watched him carefully in my mirrors. It's hard to focus on driving well when that happens.
Here's some open track car prep discussion from pro-touring.com.
http://www.pro-touring.com/forum/showthread.php?t=8701