If you need to build to 10km worth of intervals for 10km, why no 26x1600 workouts?

You don't need 25x400 sessions to run a 10,000. Not many people do that sort of thing anymore and if they do it's really not a specific 10,000 meter session, i.e., they aren't running the reps at their 10km pace. Zatopek routinely did that sort of session but most of the reps were slower than his race pace.. Lydiard's guys sometimes did 20x440 in the early part of their interval work but at relatively slow paces and more as a transition from distance and hill work.

A session of 26x1600 would have to be done very slowly or you'd pass the point of building up and begin to tear yourself down. You'd probably be running so slowly that the session wouldn't be much different from a steady run of comparable distance.

Carlos Lopes ran 15 to 20x 400m right at 10k pace once or twice every week through the year, and he hammered the last half of his road runs.

Many of the Kenyans run this type of session, but on roads and run them by time, i.e. 25x 1/1.