I would have speed and power elements at all phases of a 400/800 runners program. Many backwards thinking coaches (many of which seem to get high school jobs unfortunately) think that doing quality work early in a program gets kids hurt, when in reality, it's making an abrupt switch from all distance (many times run too slowly to begin with) to almost all speed is what gets kids hurt because distance alone (notice the term alone before you rip me) does little to get you ready for speed events and the 400 and 800 are speed events. Yes, you do need a base or a GP phase for all events, but as McFarlane says: the base for speed is primarily SPEED (sorry to yell) :yes:
I coudn't really site any formula for figuring out optimal volume, but I would say that you ought to be able to figure out what the athletes background is for a starting point. Then, I would map out your season long macrocycle with your scheme of work. I am a big fan of multi tiered training. Some endurance work is needed, but within the realm of "aerobic" work there is a wide range of running that can be truly labeled as such and it needn't be SLOW running. Slow training produces slow racing. That doesn't mean that every work out is done at max aerobic capabilities but you should be emphasizing some type of aerobic quality on most days.
Magico, you have a true grasp of training concepts as evidenced by your comment regarding 150's etc. One problem you're facing is that of ignorance of physiology by your peers (loose usage of the term, I imagine). I have 400/800 runners do some form of true speed development 40's or 60's year round. What you're talking about is wanting kids to do work at race pace early in the season and that is what they SHOULD be doing.
Somewhere in the early 80's speed became a four letter word. Some coaches were having all of their distance runners do interval type of training 4-5 days per week, plus trying to race and that was going too far in the other direction. It was breaking kids down and they weren't progressing in the 1600 and the 3200 because there was too much anaerobic work and not enough true aerobic development. So, in the US, we went the opposite direction and termed all training not done at or close to max speed as aerobic. As we all know, you need to be at or above about 60% of max to get any aerobic benefit yet you coudn't convince the less and slower is better crowd of that until you started seeing a rapid decline in high school distance performances starting around the mid 80's in the US. Ideally for the 1600 and 3200 runner to get better they would get to the point at which they could handle reasonably higher volumes AND an ample amount of quality track work at the same time before lowering that volume to induce the peak.
In an attempt to try to put some hard numbers in my loquacious answer. I probably would not have a hs 400/800 runner doing long runs of over 6-8 miles. Most aerobic training would be as Mike described above in the form of tempo running (fast continuous runs) or fartlek (intermittent surging within a medium length distance run). You need some recovery type of run days but not that many in a week-depending on how many meets you have in that week.
If I can help you in your mission to convince any coaches you work with, don't hesitate to ask. I have tried the best I can to educate as many other coaches as possible and this forum is incredibly informative so maybe you can steer some folks this way who really want to learn to help the kids they work with as much as possible because that's what we're supposed to be about.
Hang in there, it's worth it!!