This is Hal Higdons Intermediate 1 marathon schedule. Of course google it so you can see it better.
http://www.halhigdon.com/marathon/inter1.html
I would sub the ONE 18 miler at wk 10 with your first 20 miler. Believe me..... 3 miles WONT be a big jump at that point-- BUT IT WILL SUCK (the first 20 miler) Hal Higsdon does not build in speed workouts to this schedule, but you can easily tweak one of the tues or thursday runs to be track work, or tempo work or whatever.
IN MY OPINION-- i would change the MID WEEK Mid-length run to a Lactic ACid Threshold Run.... even when you are getting up to 8 or 10 miles for the mid length run.... you will reap SO MANY benefits..... you wont have to worry about track workouts
Week Mon Tue Wed Thur Fri Sat Sun
1 cross 3 m run 5 m run 3 m run rest 5 m pace 8
2 cross 3 m run 5 m run 3 m run rest 5 m run 9
3 c ross 3 m run 5 m run 3 m run rest 5 m pace 6
4 cross 3 m run 6 m run 3 m run rest 6 m pace 11
5 cross 3 m run 6 m run 3 m run rest 6 m run 12
6 cross 3 m run 5 m run 3 m run rest 6 m pace 9
7 cross 4 m run 7 m run 4 m run rest 7 m pace 14
8 cross 4 m run 7 m run 4 m run rest 7 m run 15
9 cross 4 m run 5 m run 4 m run rest 7 m pace 11
10 cross 4 m run 8 m run 4 m run rest 8 m pace 17
11 cross 5 m run 8 m run 5 m run rest 8 m run 18
12 cross 5 m run 5 m run 5 m run rest 8 m pace 13
13 cross 5 m run 8 m run 5 m run rest 5 m pace 20
14 cross 5 m run 5 m run 5 m run rest 8 m run 12
15 c ross 5 m run 8 m run 5 m run rest 5 m pace 20
16 cross 5 m run 6 m run 5 m run rest 4 m pace 12
17 cross 4 m run 5 m run 4 m run rest 3 m run 8
18 cross 3 m run 4 m run rest rest 2 m run race
this is just a good, basic blue print.... i would keep the FORMAT of the schedule, but tweak a few things, ya know? you can make some of those 5 milers into 6 milers, and i would also make at least a couple of the 8 milers into 10-13 miles..... personally..........