Pacing???

I've been doing a lot of research lately on pacing. Specifically, what pace I should be going on my long runs. I'm currently following one of Hal Higdon's training plans who recommends running your long runs at 45-90 seconds slower than your marathon pace. I understand the reasoning behind this and have mostly followed that plan. But for last weeks 17 miler I had to run "faster" than my normal LR and averaged a 9:09 pace. This run was more like a long tempo run averaging about a 9:33 pace for the first 6 miles, speeding up to average an 8:43 pace for 7 miles and slowing down to finish the last 4 miles with a 9:16 average (yes I dissected my run, excel is awesome for that :)

This has me wondering, what pace should I be running at? I don't want to run to slow but I also don't want to run too fast risking injury and over training.

A little history:
I'm new to racing only having run a couple half marathons and one full. For my first half marathon last June (North Olympic Discovery Half Marathon) I ran by feel. I wanted to finish at about 2 hours but was using the Nike plus so wasn't constantly checking my time. I was just enjoying being out there and taking it all in. I ran at a 9:28 pace finishing in 2:03. When I was done I felt like I could have kept going. That tells me I could have ran faster, I had more to give. Even the volunteer that walks you through the finisher chute to make sure you are ok said I looked great and like I should have been running the full. But I was happy with just finishing my first half. Last October I ran my first full marathon (Portland Marathon). For this race I used the same approach and wanted to enjoy it and take everything in. For the first half I felt great, even with the pouring down rain and was running at about 9:42 pace. Around mile 16 my knee started hurting really bad. I had to stop and stretch it out and tried to keep running but it was getting more difficult as the race went on. At mile 20 I was averaging a 10:33 pace. I had to run/walk the last few miles but there was no way I was going to walk into the finish line so I sucked up the pain and ran, well more like shuffled, the last 1.2 miles and went straight into the medical tent to get my knee wrapped in ice. I finished in 4:48:22 for an average pace of 11:00 min/mile. I know I could have finished Portland somewhere before 4:15 cause the 9:42 pace I had felt great and I probably would have pushed the pace more towards the end of the race.

This training cycle I have been pushing the pace more (it's easier with the Garmin) and have felt comfortable at about an 8:40 pace for my weekly runs. I averaged an 8:25 pace for my 8 mile run on Friday. Tomorrow I have an 18 miler scheduled.  So the question comes up again...What pace should I be running at?

I still haven't decided what approach I am going to use after reading the many different ones running coaches recommend. I will see how I feel and probably start it out slow and increase the pace the last few miles.

If you all have any recommendations I'm always open for suggestions to see what works for other runners.

~Always a learning runner