I hear you loud and clear on the nutrition issue, but I still cannot get my mind around that drop off and your use of the word "bonk" without thinking that nutrition may have been an issue. I suppose it could have been dehydration too, but that looks like glycogen depletion to me, especially if you weren't cramping up. You are a little bit bigger than the average marathoner, so maybe your intake just wasn't enough.
Anyway, on to the question that you actually asked, I would agree with folks who suggested more long runs and longer durations at marathon pace. I also thing that the 30%+ jump in mileage and having this stretch of training under your belt as a base will make a huge difference the next time around.
A few additional suggestions:
1. Next time around, try to get in as many runs as possible of 70 minutes or more. If you are running 16 miles easy over 2 days, instead of running 8 and 8, run it 6 and 10. This should help your body in adapting from switching between sources of glycogen while running (which happens at about the 1 hour mark) and drawing upon those different sources. Having efficient pathways is a pretty big key.
2. Consider doing quite a bit of your work at tempo pace. I found that doing sessions of 3 x 10 minutes at tempo pace with 2 minutes rest in between was a great marathon workout for me. Even being relatively untrained, that should be something that you should be able to handle with your base level of talent. I would also give some consideration to working tempo paced bouts into your long runs from time to time - maybe once every 3 or 4 weeks.