I will almost guarantee that you will do better with a huge reduction of time. The mile 19 thing is common and a difficult period for most all runners. It's that tool box thing that I talk about, having the mental tools to pull through that period.
With regard to McMillan, I find that if you are well trained, his ratios are not too far off. Though one really needs to look at training paces, as well as race paces. For example, if your sprint work or long runs are slower than the ones listed on his page, you may suffer a bit in the race. Likewise, if one has a nagging injury or the weather is different than trained in, the marks may be off.
I saw you at the Pasta feed now that I saw you photo. I thought it might be you when you walked by but by the time I knew, you were gone in the crowd.
Regardless, remember that your time is still a good time. The number one goal for a first marathon is finish - done, finish upright - done, finish with a smile - we will see in a couple days.
See you next year and GREAT JOB!