Why can't you figure it out? If you're used to running, just run as hard as you can for 30 minutes. That should get you close enough. A full-out 10k left me bordering right on the edge of a spectacular blowup over the last 10 minutes.

Anyway, 90 second 400s at threshold pace isn't a very effective workout. I'd say at least 15 minutes at a time would be necessary to get some good stuff out of it, especially at threshold pace. 400s would be closer to 3k pace or faster unless you're doing a lot of them with very short rests.