Training for 400m endurance

Here’s a progression you could use over 6-8 weeks: Total volume per session = 800-1200m - all at 400m race pace.
1. 3x150/150/100 - 30s rest on reps 6-10’ on sets
2. 3x2x200 - 45s rest on reps, 6-10’ on sets.
3. 3x2x200 - 30s rest on reps, 6-10’ on sets
4. 2x2x200 - 20s rest on reps, full recovery on sets
5. 2x250/200 - 30s/full
6. 2x300/150 - 30s/full
7. 2x350/100 - 30s/full (this one will be tough to do at race pace)
Then, if you wanted to extend the progression - you could work back down the progression going to slightly faster pace (IE - goal race pace+) and giving yourself a little more recovery between reps.