I do these sometimes. My PR is around 57 seconds (hand timed) at a body weight of about 215lbs and 30+ years of age. The last 75m were horrible and, as you allude to, an effort of pure will to keep my stride rate up. I doubt I could hit that right now. Overall, I find it to be one of the more useful training distances for my goals.
However right now my bread and butter is the 300m shuttle run (i.e. Set 2 points 25 apart, sprint from one to the other touch the line cut back and sprint to the other and so on. 12x is 300m). I find it feels similar to 400m in terms of exertion, but with the added agility elements of rapid direction changes and acceleration/deceleration. These start easy but leave you thoroughly gassed by about turn 9, in my experience.
I’ll usually do about 4 rounds of this, 6 would probably be better. Generally do this as a wait till my heart rate comes down a bit and repeat style interval, which usually amounts to a total combined work/rest cycle of around 2:30 so the whole workout (minus warm up and cool down) takes 10-12 minutes.
killerDIRK lays out some guidelines for “respectable” times at varying distances (as well as some other useful info) in the “catch your breath HIIT rest” thread going on this board. Sub 75 seconds is commonly thrown out as a decent 400m time for a reasonably fit person who doesn’t really focus on running.