I've run 137 official marathons and another 50+ races from 50k to 100 miles. I've run the same pace on the last 5 miles of my 50 mile PR as my first 5 miles that day (6:40/mile), finished under 6 hours as I had predicted and was under 3 hours at the marathon split. I was the Coach of the biggest community adult running club in California for 28+ years. Women's teams I have coached have won the Team Championship at Boston 3x in the Open and 2x in the Master's and the National Championship in the Open Marathon ....I have also lectured Sport Psychology internationally (I am a Ph.d. in Cognitive Experimental Psychology). I have permanently damaged lungs from the secondary smoke of my parents smoking, so my fastest 400m ever was only 72 seconds, yet I have run 2:41 for the marathon and 1:16 for the half-marathon. I wrote the chapter on "Training for the Ultra-Marathon" for one of the Runner's World general training books (edited by Hal Hidden).
How much do you weigh, this is also important as bigger runners NEED to run even more distance and longer distance strength building base runs. Train like a Kenyan, at least as much as your real world allows. I regularly was able to run 100+ mile weeks and up to a 10 week preparation for the WS 100 of 180 mile weeks! I was under 130 pounds at 5'9" at that time.
Long Runs....two 30 milers/50k in the 3 months before your goal marathon event. Base LONG runs on pavement 28 miles. Ideally with easy "out" and "race effort" back (Negative Split) with full race effort the last 5 miles.
Speed work....TEMPO runs of around 10 miles at faster than marathon planned pace.
Shorter speed sessions 3-4 x 2 miles at significantly faster than planned marathon pace
Ideally all "speed sessions" should be the 2nd workout of the day with a morning 6-9 mile comfortable pace session. Never do your "speed sessions" of FRESH legs. The marathon is NOT ABOUT THE FIRST 15K, but what you are ready and prepared to do over the last 10k. It is so mentally rewarding to be STRONG ENOUGH to be passing people over that last 6 miles rather than the opposite mental impact of others "sucking your energy" as YOU ARE GETTING PASSED.