Bart Yasso says that over the years he's been able to reliably predict his marathon times by his 800m times. If he runs ten 800's (with 400m jog in between) in an average of 3min 30sec per 800, then he'll run a 3hr 30min marathon. Likewise, if he runs 2:45 800's he'll run a 2:45:00 marathon.
So, through his marathon build he'll run 800's, working up to 10 in a session.
