I've been doing 20-30 minute runs mon, wed, fri. Tues and thurs i run something faster i.e. hills, stadium stairs, repeat 200s. Saturday or sunday i'll go for an easy 30-45 minute run. Sometimes it feels good to run easy and then at some point during the run i feel like i "break-through" so to speak and just get rid of all the crap in my legs and feel refreshed. I used to run xc…but if you are training specifically for a 400 i wouldn't recomend much over 40 minutes
I'll bet you run a great mile. Your training is far better suited to doing or perhaps more so the 800m than running a 400m fast.
*Note that I split this off of the original topic about Hart's training because it was evolving in to a completely separate subject.