6x1500 on the road for me, 3 min rest.
Take any 850metres in secs. gives approx marathon time in mins.If your long runs are up to scratch.
e.g.average of 6x 1500 [3min rest]=6minutes.
6minutes x60=360seconds. Divide by 15= average time per 100m in seconds=24seconds.
multiply by 8.5 to give time for 850metres in seconds 204 which is also predicted marathon time in minutes.PREDICTED MARATHON TIME IS 204 MINS OR 3HRS;24MINS.This seems to work well for middle of the packers in flat,fast Marathons.Try it.For half marathons use same system but use 800mtrs and half it.
e.g.100m time 24sx8=192 divide by 2=96min or 1hr:36mins.
this is a derivative of yasso's that is within a couple of minutes for me.My times for half and marathon with 6 min average were 1hr:36:40for half and 3hrs:22:20 for full.