When I came into college (I'm 22 and just finished with my collegiate career) I was a 4:21 and 9:25 athlete. I just tell you that to give you an idea of where I am coming from. Because as with any training suggestions, it all has to be tailored to your own strengths. You probably already knew that, so let me see if this helps you out.
I never ran much more than 80 miles per week at any time during the year because I couldn't ever handle it very well. I did at most two two-a-days, and a typical week in the summer would look something like this.
Mon-Hills or LT (The hills were always long hill reps, about 2-3 min, with 1 to 1 rest, 6 or 7 of them and LT runs were at my Lactate Threshold pace**ask if any of this sounds foreign**and I'd build up to 8 milers)
Tues-4 in the a.m.
10 in the p.m. (you can always switch when you run these. I'm just not a morning runner.)
Wed-10 (this was typically my hardest regular run of the week, if most runs were at 6:15, I'd try to get this down to 6 flat per mile.)
Thurs.-Fartlek or LT (If I ran an LT on monday, I'd do a fartlek here, if I ran a hill workout, I'd do an LT. The fartleks were long and not tapping into MVO2 range, same with hills. Just save that type of training until mid-season, if you want to know why I think that just ask)
Fri-Recovery day, but still 10, I might do another morning run. If I wanted to have Sunday off, I'd do another morning run here. This would happen maybe once a month
Sat. -16 mile long run. These runs were very important to me, and I would make them almost like another workout. I just mean that I wouldn't just go out to get them done, I would try to run them with a feeling of hard but in control. It usually wound up being around 6:15 pace, but taht pace is kind of irrelevant because I have no idea what your normal runs are at.
Sun. -I would do 4-6 if I only had one two-a-day during the week, and I'd take the day off if I'd had two. I usually shot for 70-80 mile weeks in the summer, typically going two weeks high one week lower. The lower week might come down into the 60s but not if I was feeling fine. That was usually to ward off injury.
So here's the week in short. I realize I didn't put the total milage down on the days that I was working out, so I've included that here.
Monday - 10 miles total w/ LT or hills
Tuesday - 4 miles a.m.
10 miles p.m.
Wednesday- 10 miles
Thursday - 10 miles total w/ Fartlek or LT
Friday - 10 miles w/ possible second run of 4 miles
Saturday - 16 miles
Sunday - 4-6 or off if second run was done on Friday
Total: 74-76
You'll notice that there's no way to actually get up to 80 this way. Well, the two ways that I would change that is by either going 6 on my second runs or 12 on some of the regular runs.
In general, I have always found this type of training to be very beneficial for a number of reasons. Physiology aside (because I am not an expert in that area, although I believe it is very important to consider in training) I have found that I can do this setup and avoid injury while building a good solid base. After a summer of training like this, I felt quite invincible in many areas of running, and I hadn't had to set foot on a track or tear up my calves and hamstrings with faster workouts. So, I guess what I am saying is that this set me up to do well and to stay healthy, but you are going to always have to figure out for yourself what is going to work best. I hope this has helped, and I hope I haven't talked your ear off. I'm just sitting at work, and it's a really slow day, so I felt like being a thorough as possible. Feel free to ask me to clarify something if you are confused or if I just had a typo or whatever.