Scubacane, as I said in the previous post, it's the long run that must be prioritized, especially for you, as you're struggling to maintain pace after 15 miles.
For what it's worth, this is how my plan worked out in real life. Because of injuries, I had to rewrite as I went. Every step was painful, and particularly the last month or so I needed to really back things off.
"LR2" is the secondary long run, or "medium long run," as Pfitzinger has it in the newer books. "XT" is cross training. Again, apologies to the OP for going off topic.
16 WEEKS TO GOAL
LR: 12.0 miles @ 8:00/mi
Easy: 8.5 miles @ 8:06/mi w/1mi@MP
Off
Easy: 5.1 miles @ 8:33/mi
LR2: 10.0 miles @ 7:42/mi w/1mi@MP
Easy: 4.0 miles @ 9:15/mi
Easy: 9.6 miles @ 8:28/mi
WEEK TOTAL: 49.2 MILES
15 WEEKS TO GOAL
XT: Bike 18.5 miles @ 18.1 mph
Off
LR: 13.5 miles @ 7:35/mi
XT:Bike 24.9 miles 18.9 mph
LT: 2.5-mi warmup; 2 x 2 miles w/2min rest: @6:40/mi, @6:37/mi; 2.5-mi cooldown
Easy: 4.5 miles @ 9:27/mi
Easy: 7.1 miles @ 8:20/mi
WEEK TOTAL: 34.1 MILES
14 WEEKS TO GOAL
LR: 15.2 miles @ 8:32/mi
XT:Bike 18.5 miles @ 19.1 mph
Easy: 10.0 miles @ 8:04/mi
LR2: 12.4 miles @ 7:13/mi
XT: Bike 18.5 miles @ 19.0 mph
Easy: 8.4 miles @ 8:42/mi
LT: 2-mile warmup; 4.0 miles @ 6:40/mi; 1-mile cooldown
WEEK TOTAL: 53.0 MILES
13 WEEKS TO GOAL
Easy: 7.0 miles @ 8:42/mi
Easy: 10.4 miles @ 8:51/mi
LR: 18.5 miles @ 8:02/mi
XT: Bike 18.5 @ 18.1 mph
Easy: 6.0 miles @ 8:29/mi
LR2: 12.0 miles @ 7:07/mi
XT: Bike 18.5 miles @ 17.9 mph
WEEK TOTAL: 53.9 MILES
12 WEEKS TO GOAL:
Off
LT: 2-mi warmup; 5.0 miles @ 6:40/mi; 2-mi cooldown
Easy: 10.4 miles @ 7:56/mi
Easy: 10.4 miles @ 8:50/mi
LR: 18.1 miles @ 7:33/mi
XT:Bike 14.4 miles @ 18.5 mph
Easy: 6.0 miles @ 10:42/mi
WEEK TOTAL: 53.9 MILES
11 WEEKS TO GOAL
XT:Bike 23.8 miles @ 18.6 mph
LR2: 14.0 miles @ 8:24/mi avg (progression 10:32/mi to 6:54/mi)
XT:Bike 26.8miles @ 18.5 mph
Easy: 3.0 miles @ 9:31/mi
LT: 2-mi warmup; 3 x 2 miles w/2-min recoveries @6:36/mi, @6:33/mi, @6:30/mi; 1.5-mi cooldown
XT:Bike 35.3 miles @ 17.6 mph
Off
WEEK TOTAL: 26.5 MILES
10 WEEKS TO GOAL:
LR: 20.0 miles @ 8:54/mi
XT:Bike 22.9 miles @ 18.3 mph
Easy: 7.0 miles @ 8:35/mi
LT: 2.5-mi warmup; 5.0 miles @ 6:45; 2.5-mi cooldown
XT:Bike 18.3 miles @ 19.1 mph
LR2: 14.3 miles @ 7:42/mi
Off
WEEK TOTAL: 51.3 MILES
9 WEEKS TO GOAL
Easy: 6.0 miles @ 9:29/mi
Easy: 5.0 miles @ 9:52/mi
LT: 3-mi warmup; 5.0 miles @ 6:40/mi; 2-mi cooldown
Easy: 7.0 miles @ 8:22/mi
LR: 12.0 miles @ 8:14/mi
XT:Bike 27.4 miles @ 19.0 mph
Easy: 7.0 miles @ 8:31/mi
WEEK TOTAL: 47.0 MILES
8 WEEKS TO GOAL
Off
LR: 20.0 miles @ 7:56/mi (1-10) @ 7:12/mi (11-20)
XT:Bike 18.3 miles @ 18.7 mph
Easy: 10.0 miles @ 7:46/mi
LT: 2.5-mi warmup; 3 x 2 miles w/2min recoveries @ (6:31-6:29), (6:29-6:32), (6:30-6:20); 2.5-mile cooldown
XT/Easy:Bike 27.4 miles @ 19.0 mph; 5.0 miles @ 8:54/mi
Easy: 5.0 miles @ 8:48/mi
WEEK TOTAL: 50.0 MILES
7 WEEKS TO GOAL
XT:Bike 23.7 miles @ 18.8 mph
Easy: 10.0 miles @ 8:40/mi w/6 x 90-sec @ 5K pace
XT:Bike 18.3 miles @ 19.8 mph
Easy: 7.0 miles @ 8:47/mi w/3 x 90-sec @ 5K pace
XT:Bike 14.5 miles @ 18.5 mph
Easy: 3.0 miles @ 9:38/mi w/1 x 90-sec @ 5K pace
LT: 2-mi warmup; 15K race @ 6:26/mi
WEEK TOTAL: 43.3 MILES
6 WEEKS TO GOAL
Off
XT:Bike 14.5 miles @ 19.2 mph
VO2max: 1.5-mi warmup; 4 x 880y w/2min recoveries @ 3:05, 3:07, 2:56, 2:57 1.5-mi cooldown
Easy: 5.0 miles @ 9:32/mi; bike 7.0 miles
LR: 18.0 miles @ 8:31/mi
XT:Bike 18.3 miles @ 18.5 mph
Easy: 7.0 miles @ 8:55/mi
WEEK TOTAL: 30.0 MILES
5 WEEKS TO GOAL
Off
XT:Bike 73 miles untimed
Easy: 5.0 miles untimed
LR: 20.0 miles @ 7:45/mi
XT:Bike 18 miles untimed
VO2max: 1.5-mi warmup; 4 x 1000m w/3:15 jogs @ 3:49, 3:44, 3:43, 3:40; 2.5-mi cooldown
Easy: 10.0 miles @ 8:43/mi
WEEK TOTAL: 41.7 MILES
4 WEEKS TO GOAL
Off
Easy: 8.0 miles @ 8:41/mi
VO2max: 1.3-mi warmup; 5 x 800m w/2:10 walk/jog @ 2:56, 2:56, 2:55, 2:55, 2:54; 1.0-mi cooldown
LR: 18.3 miles @ 7:43/mi
XT:Bike 23.8 miles @ 19.7 mph
LT: 3-mi warmup; 4.0 miles @ 6:35/mi; 2-mi cooldown
Easy: 7.0 miles @ 10:58/mi
WEEK TOTAL: 47.5 MILES
3 WEEKS TO GOAL
XT:Bike 18.3 miles @ 18.6 mph
VO2max: 3.5-mi warmup; 3 x 1 mile w/4:55 walk/jog @ 6:16, 6:13, 6:05; 2.5-mi cooldown
XT:Bike 23.8 miles @ 19.7 mph
Easy: 4.0 miles @ 8:41/mi; Bike 24.0 miles @ 19.6 mph
LR: 22.5 miles @ 8:08/mi
XT:Bike 19.8 miles @ 18.7 mph
Easy: 5.0 miles @ 10:12/mi
WEEK TOTAL: 40.8 MILES
3 WEEKS TO GOAL
Off
Off
VO2max: 5.0-mi warmup; 5 x 880y w/2min recoveries @ 3:08, 3:07, 3:06, 3:02, 3:03; 6.5-mi cooldown; Bike 18.3 miles @ 18.4 mph
Off
LT: 2.0-mile warmup; 3 x 1 mile w/90sec recoveries @ 6:32, 6:24, 6:22; 1.0-mile cooldown
Off
Off
WEEK TOTAL: 20.0 MILES
2 WEEKS TO GOAL
LR 20.0 miles @ 8:07mi
XT:Bike 20.1 miles @ 19.5 mph
Easy: 10.0 miles @ 8:28/mi
XT:Bike 18.7 miles @ 19.2 mph
LT: 48-minute run with 20 mins at perceived threshold pace
Off
Easy: 40-minute run, very slowly
WEEK TOTAL: 40 MILES
1 WEEK TO GOAL
Off
Off
Easy: 4.6 miles @ 8:24/mi
Easy: 7.0 miles @ 7:51/mi
Off
XT: Bike 32.5 miles @ 18.6 mph
Easy: 5.1 miles @ 7:26/mi
WEEK TOTAL: 16.7 MILES
RACE WEEK:
Off
Easy: 4.0 miles (splits - 8:48, 7:55, 7:49, 7:15)
Easy: 4.0 miles (splits - 9:03, 8:21, 7:57, 6:53)
Easy: 4.0 miles (splits - 8:22, 7:49, 7:07, 6:44)
Easy: 2.0 miles @ 7:54/mi
Off
Race: Marathon 3:07:23 (7:09/mi)
WEEK TOTAL: 40.2 MILES