400/800 is a strange one. Here's a quick plan to get you started until track season starts. I really don't think you need to concentrate on special endurance or really any kind of race pace stuff until the start of the season but different people will say different things:
4x40y(full recovery), 35 min easy run
plyo, 35 min easy run, lift
4x120y(full recovery), 35 min easy run
plyo, 35 minute easy run, lift
40 min easy run, lift
70 min easy run
as you get close to the season, i would replace the Wednesday workout with something more intensive, here are some examples:
4-8x200 with minimal recovery (good for 800)
4x300 with long recovery (good for 400)
8x200 @ race pace with minimal recovery (great for 800, tons of race pace work will really really help you in the 800)
3x400 with 10 mins recovery (good for 400 and 800)
you can even mix in slower pace stuff with the fast stuff - this might help you, considering you described yourself as more distance-oriented guy. here is example:
4x150 with full recovery + 6x300 @cv pace with 1 minute recovery
if you decide to pursue 400/800 during the season, I would replace the saturday long run with long reps on the track instead when you get to early season. something like 5x1000m @ cv/threshold pace.
- 400/800 training resembles sprint training more than distance training.
- this means consistent lifting (work on being explosive) and plyo to develop that fast twitch. i've always been told to train like a boxer but you do you.
- current mileage should put you at around 35 mpw. imo, that's pretty much the sweet spot for someone of your profile, but do what feels best.
- do what works best. i've heard of people running sub 1:50 off of 90 miles a week, and i've heard of people doing the same thing without running a rep longer than 300m on the track. again.