I wouldn't agree with not training at fast paces during base building. During the Summer, I used to hit every pace from sprint to recovery. I would have a tempo every week at about 10-20 seconds below lactate threshold or I would alternate with a fartlek session. I would also have a speed development day every week consisting of 150-2000m repeats done with full recoveries. An example would be when I was in peak shape, 12x400 at 70-72 per rep with a full recovery(just slower than 5k pace. 6-8x800 at 2:28-2.32 with full recovery and 4-6 1600m at 5:10-5:20 with full recovery. Full recoveries are untimed and you should take as long as you like. An example for me was, during a 400m repeat session, 90-120s recovery between reps.

You don't want to accumulate Lactate in the base phase but you should still be working on your neuromuscular and mechanics. The key to doing this is to add speed and intensity but keep the recoveries long so you don't accumulate any lactate which leads to early peaking. Base training like this ensures you will have speed when you transfer into the specific mesocycles from base, you will have an extra gear come goal race. None of the workouts should leave you drooling on the ground because they are done easier than your race specific phase but you want to keep in touch with all paces during the base phase.

For the shorter reps from 150-300m would be broken into thirds. The first third of the rep would be acceleration, the second third at close to top end speed and the final third was a stride with full recoveries. I had two workouts a week, one day was repeats and another day was fartlek/tempo.

It's a common misconception that the base phase is all slow easy running, It should be all easy but not all slow and easy. Those reps and tempo's I mentioned above are all easy workouts but they are far from slow. They do not cross the lactate threshold because of pace and the length of recovery.