Yasso 800's work best for the purpose for which Bart Yasso intended them. That isn't
so much as a training method, but as a test tool to verify one's specific marathon goal. (However, it isn't the only test
that should be used.) Specifically, if you can't complete 10x800 at "Yasso pace", then you probably can't run your marathon
goal. OTOH, just because you can complete 10x800 at Yasso pace....or faster....doesn't necessarily mean that you are fully
prepared for your marathon goal.
Achieving a particular marathon goal requires adequate endurance, anaerobic threshold
(AT), aerobic capacity (VO2max), running economy, and running strength. Endurance comes from total mileage and long runs.
(The others are developed through speed work, hill running, weight training and short distance racing.) Of them, AT is most
important for the marathon distance. However, Yasso's as a primary speed training tool isn't the best way to develop AT. Tempo
runs, cruise intervals and 10k-half marathon races are. Yasso's, which are specifically 10x800, are primarily an extended
VO2max workout. A VO2max workout consists of 400-800 meter intervals at 5k race pace or a little faster. The "Yasso pace"
of marathon time goal translated to minutes:seconds is within 3-5 sec/800 of 5k race pace.....close, but not quite optimum
for VO2max training. (It's as much as 10/sec/mile slower than VO2max pace.) It's the number of intervals (10) in a full set
of Yasso's that make it a good VO2max workout/test. However, running fewer intervals at Yasso pace while building up to 10
is less than an optimum VO2max workout. I would suggest running fewer than 8x800 at 5k race pace, which should be a little
faster than Yasso pace.
For marathon speed training purposes, I think it's best to include a mix of AT runs
and VO2max intervals (slightly faster than Yasso pace), plus a few hard repetition and hill repeat sessions for running economy
and strength, than it is to focus on partial sets of Yasso’s as primary speedwork.
A full set of Yasso's (10x800 meters) run 2-3 weeks before the target marathon should
be one last input to finalizing a race goal....but not the only one. Prior to running them, a goal should be established based
on performance in a shorter distance race (10k-half marathon) run 3-4 weeks before the marathon, plus the "comfortably hard"
pace of LSD's and AT workouts. The final set of Yasso's should be the final confirmation of the goal.