Stupid question about training (proportion of race distance via workout)

Of course you can run further at a slower pace.

This is a good question actually. I think it just has to do with rep length which dictates how well you're able to recover after each one.

Intense 400s are quite taxing, and even moreso if you're doing a lot of them, but since they're short you can increase the rest time to allow for ample recovery. It's harder to do that with threshold since you're either doing it continuously or in long intervals, to where if you lengthen the rest too much proportionally then it's harder to maintain the desired HR in between each rep. Physiologically, it would change the stimulus of the workout.

I think in those two examples with the 10 x 400 vs 60 min at threshold the training goals differ too. A lot of the time with race pace work, the pace itself is more important than the rest as long as it's within reason.

Also, for what it's worth, predictor workouts usually need to be slightly higher in total volume than the target race distance in order to be accurate. In general, workouts for a specific distance are higher volume.

You can absolutely do more than 5000m of 5k pace in reps - 4 x mile @5k is common and I've done it, actually I've done 5 reps with 3 min recovery but it's a very hard workout to do week after week. Not sure about 10k workouts but I think 12 x 1k would be pretty doable