Bingo! http://www.coachbenson.com/images/8090.pdf This Coach Benson site is great. This particular chart gives you the 10K time (51:00 min/8:14 pace) you need to be running, in order to run a realistic sub-4 marathon, without stopping. It also suggests that 77-78% effort is where you need to be for running 26 miles--that's about a 9:05 to 9:09 pace. With walk breaks and congestion of a big marathon, I think we need to be around 8:40-45. (Roy, tell me different???) So move up the middle "pace" column to 8:45, slide to the left, and you can see that your 10K time needs to be 49:00 min/7:54 pace. Those are probably pretty good targets, assuming all sorts of things on race day: cool weather, no injuries, max 20 second stops per mile average including bottlenecked turns, no excessive pissing, blood on the highway, no stopping for a beer in Wrigleyville, etc.
For pacing (effort) on other runs (like recovery or intervals), see chart in earlier post on Effort Based Training. I really think these charts give you a model to gauge effort without overtraining. Keeps you focused on the gradual build. Yet, it provides goals to hit. I'd really like to see the pacing information above related to 5K times as well.....maybe it's too short of a distance.