Yasso 800's is quite popular amongst marathoners for determining potential marathon times. It is a simple workout which says if your goal marathon pace is 4:00hrs then you can reach that goal with 800m repeats done at 4:00 min pace(i.e 8mns per mile). More specifics on the workouts is listed here.
I tried Yasso 800's during my 2010 CIM marathon, I solely did the workouts for 8 weeks , my speed workouts were mainly (8-10)X800M in 3:30 and I did feel good improvements in my VO2 max. I PR'ed in that race time: 3:38 hrs.
But based on my current training for 2011 CIM and experience of few of my running friends I have seen it to be little optimistic predictor by say 5-10 mins. i.e if one does 10X 800m workouts in 4mins, more likely runner is capable of doing marathon in 4:10.
I suggest a simple enhancement to this workout, to prolong these workouts to 1200m instead at the same pace. which I have found be much closer estimator, provided your long runs are inline with your recommend pace.
how it works:
For a goal marathon time of 3:20, your 800m time would be 3:20 mins, 1200m in 3:20 x 1.5 = 5:00. Try doing each of the 1200m run in 5:00mins. After every 1200n, do a recovery 400m lap in about the same time it takes to do 800m. Repeat this for 6 to 10 times during the last 8 weeks of your marathon training . I think this would come much closer to your goal marathon time. Also one would maximize on the benefits of VO2max training by doing for such a duration. According to Pete Pfitzinger to get benefits from VO2max workouts, each of the repeats should be done for 2-6 mins and a distance of 1200m seems more optimal.
Try this on your next marathon and see how close you get to your marathon.
If target marathon time is not known then a average of 8 X1200m should be able to indicate your target marathon time. i.e if 1200m is done in 5mins 30secs (800m in 3:30mins), one is capable of a 3:30hr marathon.