How can I improve my 400m time?

i like clyde hart's (johnson and wariner's coach) ideas. look up his workouts. it's good speed meets the ability to endure at that speed longer than the next guy. so a mix of stereotypical windsprints, but also ladder stuff or longer workouts where you have to fight being tired, longer intervals out to 500-800 where you are running past your intended finish line trying to keep it together, and some outright hills and light cross country work.

to be clear, he still thinks speed tops endurance, as you can train a speedster to endure but how quick you can go limits the split capabilities of a more endurance type. you can work on getting a 22 200m guy stronger. you can't make a 25 200m endurance kid faster and that limits their possible 400m.

so favor speed. my personal experience once i was fit my sprints would drop together in lockstep.

re your question, he varies rest depending on the workout type and the effort expected. if you are going full gas and trying to produce afterburner work at near meet pace, long rest. but some of his workouts are short rest and designed for oxygen debt.

anyhow, there are hart workout plans PDF'd online.

i shouldn't say "can't" re the endurance runner. some might get faster. i mean more like some people are designed for distance and hit their ceiling. and they hit a time limit. and that dictates your 400m won't go below a time if you know the conversions. eg some guy who was on here who could barely break 30 200m wanted to break 60 at 400. like you probably need to near the mid 20s to get the 400 under 60. you can't full gas the whole thing and you're going to fade somewhat. so any goal involves getting the sprint number down plus the endurance to hold as much of the split as possible. which even for the good ones is a pace that works out like their (200 pace)2 +3 to 5s. so you can see where the sprint speed sets a ceiling since you're also going to fade from that.

For pure speed; low volume workouts with short sprints & full recovery. For speed endurance; higher volume workouts, either short sprints with incomplete recovery or longer sprints with partial to full recovery, but you want to be recovered enough so that you're still able to have good sprint form. Speed endurance workouts for 400m runners are often going to be higher volume and slower (closer to 400m race pace) than workouts for 100-200m runners.