There were a couple of active threads about 1 year ago when Charlie wrote about some of this, also some in CFTS and Vanc04.
Charlie said that if you are coming from the type of SPP1 given in Vanc04 where you are doing things like 3X4X60 or 4X4X60 for SE and maybe 4X60 with long rest for speed, you take a week off after indoor season, then you move up to 2X3X80 (rests 7-8 min/15 min) and maybe 3X80 with 15 min (or flying sprints) for speed. The 80’s start the true alactic work, so the rest periods are significantly longer. You stay with the 2X3X80 for a couple or a few weeks before moving up to 100’s and 120’s (not more than 3-4 reps/session with long rest).
I actually did this last year, where the workouts in the 3-1-3 for SPP2 were 3X3X80 and 3X70, then moving to 3X120 and 3X80 during precomp. What I learned was that I wasn’t able to get my speed endurance in line with speed until Phase 3, and by then I had essentially run out of races to run in. So this year I moved to start the longer SE earlier and have some special endurance in Phase 2. The 3X3X80 actually came during precomp (tapering) for indoors this year, moving to 30+3X100 during indoor comp (see CFTS about indoor), then up to 120 during GPP2 (only 2 weeks in length per CF), and I have workouts with reps as long as 4X150 and John Smith’s 300-200-100 during SPP2, and this seems to be working much better.
When you can move to 2X3X80 is going to depend on your training age and the type of prep work you have done. You don’t move up in length until you can maintain the pace of the SE work, but you want to move up to the true speed endurance (80-120 or 150 with long rest) as soon as you are able to handle it. The sooner you can get there, the more likely you will be able to hold together the last 30 meters of your 100 before the end of the season.
One more point here is what Charlie has said about pulling back weights to provide for speed endurance. The weight sessions are pulled back to just 2X6 during SPP, and the reduced load in the gym allows for more load available at the track.