Thanks for writing in with your questions. Your race falls below the normal distance we cover in Training for the Uphill Athlete so we have to read between the lines a little bit. At 21km and 3.5 hours this race is closer in average intensity to a Skimo Individual race. More intensity is probably advised.
The ME and strength along with aerobic base work should provide a good base to launch into more event specific work in the specific weeks. You probably will benefit from a second high intensity training session as is recommend on p301 under “High Intensity”. Be cautious that you do not drop the volume of the aerobic base work too much in these specific weeks. It is that aerobic base that plays a critical role in supporting the high intensity and the race itself.
We along with many others have found that the use of Zone 4 intensity and a more polarized approach to the training where easy means EASY and hard means HARD nets the best results in terms of race performance. Z4 work does everything a Z3 session does physiologically but with an increased cardiovascular and ME load. I have used Z4 interval session with several ultra long distance professions to very good effect. In those weeks, which follow the base period with its ME, we focus on running has hard uphill for anywhere between 6 and 12 minutes and repeating this for the prescribed volume. However, there is nothing grossly working with having 1 Zone 3 and 1 Zone 4 workout in the week. Especially if you have had good results with that.
Refer to the quote on page 170 from Per Astrand. Essential “No one really knows the best method for improving endurance”, which is why there are so many variations for applying high intensity training. This may be changing: Stephen Seilier has recently conducted studies on nationally ranked and World Cup Norwegian Cross Country skiers comparing the results of different interval protocols 4min (done in upper Z4), 8min(lower Z4) and 16min (in Z3). The intensity was regulated mainly by the length (Go as hard as you can for the selected time). Z3 was all that could be sustained in the 16min work periods. The results of his test showed that the 8min in Z4 was the most effective at improving performance and certain physiologic parameters.
In the end, when it comes to high intensity use individual experience by athletes and coaches will probably dictate the recommended approach.