Sounds more like dehydration than anything else. I've had this happen often during my long runs as sometimes it's difficult to take on or find sufficient water. Shouldn't be an issue on the big day, so long as you drink often.
When doing long runs like this it's sometimes helpful to do circuits where you can regularly pass somewhere to replenish your supply.
Despite having a couple of HRM's I still do my running by feel. Recovery pace tends to vary from 8:00 to 7:30 pace. Easy & Long runs around about 7:30 pace. Steady runs 6:50 to 7:10 pace. A lot depends on the terrain and conditions. I do sometimes use the HRM to check how accurate my "feel" is.
When I was in Paris last month on my honeymoon I said to my wife, "This is a beautiful place. Do you fancy coming back here early April?" Her immediate response was, "Is there a marathon on that weekend?"...no Paris marathon for me in 2008. Not doing FLM either so am currently considering Lochaber or an autumn one instead.
Today: What: 5 miles easy Why: Felt tired after round of golf and want to do a steady 14 miles tomorrow.
What: 8 miles including 5k TT Why: haven't been feeling great recently and wanted a comparison with this time last year. Ended up doing 18:09, which was a few seconds better than last year. Pleased that I was able to push myself to the limit with only a few beats off my MHR at the end.
Has anyone else noticed that their 5kTT pace is pretty similar to their 10k race pace?