I am of the opinion that it was your nutrition strategy that caused you to hit the wall at 18 miles.
Some research has been done which shows that most marathon runners who hit the wall do so within 2 miles of their longest run distance.
That you totally changed your nutrition straegy for the marathon did not really bode well. That you only slowed by 1min/m shows that you didn't get it too wrong. That you felt bad afterward suggests that you did indeed take too many gels.
You need to do what your body is used to and for you that is minimal nutrition during your runs. You would probably do better by only sipping water when you feel the need until about the 16 mile mark which should give it enough time to work before you hit the wall.
Nothing to stop you going out a little slower and trying for a negative split, continually asses how you feel, hard because you'll be feeling great at the start but you need to hold back.