Looking for a little advice from marathon vets.
In early March, I began training for a half-marathon that was on June 3. My goal was to run a sub-2:00 half, and I barely squeaked in ahead at 1:57:58. Being my first-ever half, I realized afterward that there were a lot of small improvements I could make to cut down my time. I ran my second half-marathon this past weekend, and the tweaks and additional training got me across the finish line at 1:50:56, despite it being off-and-on rainy the entire time. I was able to finish strong and felt good the rest of the day Sunday and on Monday, rather than crossing the finish line and feeling like crap.
As I approached my first half in the spring, I decided to sign up for the Chicago Marathon on October 7. This will be about 30 total weeks of training from early March, and 18 weeks from the date of my first half on June 3. Given my time this past weekend, the fact I know the Chicago course very well (the half this weekend was basically a truncated version of the full marathon course), and the fact I have another 11 weeks of training, it seems like running a sub-4:00 marathon is a somewhat realistic goal, particularly if the weather cooperates a little more than it did this past weekend.
Thus, where I'm seeking advice. From a pacing perspective, everything I have read advises against splits that vary significantly during a marathon. That said, I'm a little uneasy with the idea of starting off at a 9:09/pace and assuming I won't slow down at all during the run. Without knowing how I'll react to the wall, is pacing for a 3:55 marathon a good approach? How much should my splits vary: would something like 8:40/mile the first hour, 8:50/mile the second hour, 9:00/mile the third hour, and 9:10/mile the rest of the run too dramatic of a pace difference between splits? Given that an 8:28 over a half felt fine, would finishing the first half in about an 8:45 pace keep me energized enough for the back half?
Part of me also wonders if I should aim for getting to 14 miles in the first two hours (8:34), allowing me to slow down pretty dramatically the last 12.2 miles. That idea of "banking time" seems to be what everyone advises against, though.
Most articles I've read about the subject are incredibly generic and not very helpful, so any thoughts/tips would be helpful. I've had a few runs with mileages in the mid-teens and felt fine (with gels and lots of water along the way), but getting past 20-miles will be pretty foreign to me- even come race day. Naive question, but is the "wall" real, or is it largely indicative of someone who hasn't trained/prepared/ate/hydrated enough?