The pace on "Yasso" 800s does typically fall somewhere between the optimal critical training zones. The efficacy of the Yasso 800s vis-a-vis other workouts will depend in part on your own recent running background (read: mileage) and on your basic abilities (i.e., do you have more talent for middle distances or are you a natural long distance runner?). As a general rule, though, Yasso 800s are slightly too fast relative to the pace which brings you to your lactate threshold (LT), and they are too slow relative to the pace which elicits your VO2max. If you run closer to your LT pace or closer to your VO2max pace, you will get a more cost-effective workout, provided you spend enough time at pace AND provided you control the lactate accumulation during the workout.

 In case you didn't know, a decent estimate of your LT pace is that which you could run for a one hour race (using an EVEN pacing strategy, of course). A decent estimate of your VO2max pace is that which you could run for a 10 minute race. Of course, this ASSUMES you are already trained well enough to run pretty hard for an hour and that you're capable of really hammering it balls-out for 10 min.

 In practice, you will usually exceed your LT (and will subsequently begin breathing noticeably harder or faster) after 18-22 min. at your LT pace. In Ex. Phys. parlance, the point of hyperventilation is called the Respiratory Compensation Point. Even though your blood lactate levels begin to rise faster at this point (actually, BEFORE this point), you can still maintain the pace for the full hour (give or take a bit) by relying more and more on anaerobic glycolysis (energy production) as the race progresses. Anyway, it turns out that 20-ish min. at LT pace provides optimal stimulus for improvement if you are running CONTINUOUSLY at that speed, and 34-42 min. at or just below (slower than) LT pace provides optimal stimulus for improvement if very short REST PERIODS are incorporated. All this has been determined and refined by trial-and-error, not by theory, so you can rely on it.

 At VO2max pace, 15-20 min. spent at or very near pace delivers the best results. It takes between 3 and 4 min. to achieve VO2max from a dead start if you're running evenly at this speed, but you can attain VO2max repeatedly during the middle and latter stages of a workout of repetitions as short as 1-2 min. if you orchestrate the rest periods correctly. Ideal rest periods for VO2max workouts are slightly shorter than the length of the previous run period (e.g., 2:00 at VO2max pace, 1:45 walk or 2:00 jog, repeat). If you rest longer than you run on this sort of workout, you'll actually accumulate MORE blood lactate (that's bad). This has to do with the phenomenon known as "venous pooling", which undermines the ability of the heart, liver, kidneys, and non-working skeletal muscles to take up the blood lactate and re-metabolize it. So bear that in mind - too much rest is counterproductive when doing a VO2max workout.

 Anyway, the repetition running I'd recommend in lieu of Yasso 800s includes the following workouts.

 LT workouts:

 1.) 12 x 3 min. at your estimated LT pace, with 30-60 secs. rest periods between each (just start the next rep when you're ready, and as you get fitter, you'll get closer to 30 secs. rests). This workout is MOST effective if you select a pace which will allow you to run the FASTEST reps at the END of the workout without undue struggling (it should feel like a good, strong rhythm run that you'd do on the road if you felt good).

 2.) 15-20 x 1 min. STARTING at your estimated LT pace (on the first 1-3 reps), slightly FASTER during the middle of the workout (get into a good rhythm here), and progressively faster still on the last 2-3 reps, with 20-30 secs. rest periods. Since those rests are so short, you'd better select a starting speed that doesn't get you in trouble early on, or you'll make the workout too anaerobic and it won't be as effective.

 VO2max workouts:

 3.) 6 x 3 min. at your estimated VO2max pace, with just under 3 min. rest periods (some or all of this can be jogged).

 4.) 12-15 x 1 min. STARTING at VO2max pace, slightly faster in the middle of the workout, progressively faster still over the last 2-3 reps, with 55-60 secs. rest periods.

 5.) 5 x 5-6 min. at about 95% of your VO2max speed (about 15-20 secs. per mile SLOWER than VO2max pace), with roughly 3 min. rest periods between each (very easy jog on most of this rest period).

 Give those a try. Obviously, you've got to be reasonably fit be-FORE doing sessions such as these, AND you must cover all the other bases in your training (recovery runs, long runs, tempo runs, a few short races here and there, etc.). This is just GENERIC information and it DOESN'T tell you how to fit the workouts into an overall program. You'll have to be smart about that. If you do want to do the Yasso 800s, try doing them with rest periods of about HALF of the run periods rather than EQUAL to the run periods.

 Train hard and smart, but above all, enjoy.