Both sessions are pretty tough. The 20x200 is a workout that you can “learn to do” early in a cycle and make you think you’re really ready to race after you’ve done it a couple of times. But if you don’t have the background work in you’ll end up going out too fast and blowing up.
The 5-4-3-2-1 is much harder to fake your way through and is a much better indicator. Depending on the rout that you run it on, if you can average 5:15-5:20 ish pace for the session, you’re good to break 17:00.....at least that is about how it’s worked out for us doing the session on a slow loop and then racing on a fairly slow course.
Both sessions require you to be in pretty good shape and both carry a pretty high risk of blowing up mid way through. If you want to get the most out of them, you should be in pretty good shape before attempting. Both are also geared to give you that “oh sh!t” feeling pretty early on and teaching you to deal with it. Most high school races go out stupid fast. At our section meet course, we have to cross a levee only 300m into the race and every 20 minute 5k kid runs that first 300 in 15:30 pace, then spend the rest of the race slowing down. For us we can go out too fast and still be buried behind 150 runners, 110 of whom will slow down after about 3 minutes and clog up the trail. so the sessions for us are aimed at allowing us to go out over our heads, far enough up in the field, and then not die in the last mile.
Not the same situation in a road race or a track race, but the physiological benefit of developing lactate tolerance is still something that will help you with your racing. Usually I avoid posting things that indicate that there is a magic workout or simple adjustment to get someone to their goals, but like I said earlier, I think your fitness is there already.
How do we get to where we can do those sessions? Early in the training cycle, we do lots of 200s with adequate rest. Not lots as in all in one workout, rather we do 4-6 of them usually twice a week. Usually they are tacked on at the end of a cruise interval or tempo session or as our pre meet workout. As we get further along, we chop the rest down to 30 seconds, but keep the same pace. Late in the season, if the schedule allows, we’ll throw in 1-2 of the really high lactate sessions described above.