I didn't set out to do this workout....but ended up doing something very similar to what Lydiard found...i found the same results however. By running long one day (up to or even over 20) and going shorter the next day but faster - more like a steady state (8-12) miles...I was able to recovery and repeat this 2 day cycle.
I know many on this topic believe doubles are better than singles (and there are several threads on it in great detail) however, by breaking all this milage up into 2 runs per day - you'll notice your longest run is only going to be 12.5 miles (oops i just saw the 15 miler in there - that is much better). i think you have a pretty good plan, but would still like to see your Tues and Thursdays focus on 1 longer run. Since you are already doing doubles, i would not say stop doing them...but i would encourage you to take that one day of 10/10 and just do one longer run - try for 15+. and go shorter on Thursday's first run if you are going to go longer on the second.
The goal of training is not to log miles...It should be to build up your endurance first (which i believe is what Lydiard suggests).
I bumped my milage from 70-80 range to 100-115 by doing this - these are all singles for me:
Sun - 20+
Mon - 8-12 SS
Tues - 20+
Wed - 8-12 SS
THur 8-12
Fri - 20+
Sat - 8-12 SS
I did not start out at 20 however...i was probably doing more like 14, 14, 12 during the first week and built from there.
Being a high schooler you should be aware that you might not be able to do 3 long runs in a week. Most on here will tell you to only to 1 every week to 10 days and that is pretty much the guide i used for years and years until i decided to experiment...and i found that i can do 3 long runs per week (during my base phase) and have no trouble recovery from them - but i think you should know you can not be running these long runs fast at all. They should be very easy paced so that you are not sore the next day. you may be a little tired the next day, but if you are sore you ran them too fast. I started around 7:40 pace in the middle of summer...but they started to natuarally get faster as the weather cooled...now i run them close to 6:30-6:40 but the effort is still very easy. Anyway the actual pace is not important - going slow and feeling easy is...actaully the slower the better for me...i was actually happy when i saw how slow i could go because i had always run these too fast - by slowing down you are forcing your body to learn how to burn fat which if you have not been doing slower running will not be easy.
don't get obessed with hitting 100 - the real goal should be on just doing the work - the longs are the key intially..the in between days are likey to just be recovery for you for the most part. Once you have succussfully adapted to this volume you could begin to add a steady state here or there (but start out easy - the goal is to just hold a nice pace)