Just finished my first 40k. Out and back course, nearly completely flat. It turns out I suck royally at pacing -- started out way too hard. There was a slight headwind on the way out, which made it not the worst thing in the world.
Just after the turnaround, I started getting the chills (temperature was in the mid 80s). I ended up going through about a liter and a half of water (camelbak thing), and felt reasonably alright about five minutes after I finished, so I'm not quite sure what the deal was.
Anyway, you can see how sharply the power started to drop off toward the end. Average heart rate for the effort was 200bpm on the nose (I'm 23), peaking at 206. It definitely sucked.
Good enough for 5th in the 4s, unofficial time of 58:29. I was shooting for 57, but oh well. Winner in the 4s did it in 54:something. Slightly ludicrous.