hotpan,
If the course is flat and you want to see how close to 33 minutes you can get, see where the race sets up in front of you before deciding to go out in 16:40. 16:30 won't kill you the way you've been running. a lot of strength runners don't do repeat kilos in sub 3:10 so they should never go out in under 3:15 but if you've got that much recent work in at faster paces, you'll be better off racing at 3:18 than running alone at 3:20.