Long runs during MRT are essentially rehearsal for race day, so you want to try out various things to see what works for you and what doesn't.

For me, dinner night before is something like chicken and rice, meat and baked potato, pasta, etc. I try to avoid anything that is really heavy or fatty (pizza didn't work well) or overly spicy or high fiber (chili wasn't great the next day either). I don't carb load per se, but I do make sure to get some good carbs with dinner.

Breakfast is coffee and cereal or a bagel, with maybe a banana half an hour before I go out if I'm hungry or feel I need some energy. I usually wait about 2 hours between breakfast and my run. During the run, I eat a gel every 45 minutes or so. Again, this is rehearsal for race day, and also to get my stomach used to that sudden influx of sugar. I usually drink every two miles or so, again, part of getting used to race day. I used to spread my gels out more, but I found that I ran out of steam badly at the end of my run if I only took one every 6 miles. That works on shorter runs, but when I get above 15 miles, I need more fuel.

I'm training for my 7th full marathon and I struggled with the same things at first. I think I've figured myself out though: My run-day breakfast consists of a half of a banana, and a bowl of plain oatmeal (instant because it's so easy!) and I stif in a half scoop of protein powder. Oatmeal is great because it is a "slow-release" carb so you'll benefit from it longer. I also drink a small coffee. During the run, every 6-8mi I will have either GU with caffeine or Powerade chews. If it's really hot, I will also intake some salt tabs and some very diluted electrolyte drink. I find if it's too concentrated I get stomach cramps. It's important to fuel before you bonk to prevent running out of gas so try to figure out what your body needs and stick to that schedule. The day before a long run, I generally eat my carby meal no later than 3pm to prevent gastro-issues while running. This could be a rice dish, or a gluten free pasta dish - never anything deep fried. In the heat, salt is a runners best friend - it's only dangerous for people with hypertension, so don't be afraid to salt load the day before a run as well to prevent dehydration.

Aside from food, start your training runs at an easy pace and work up to your intended pace. If you start out too fast that can also affect your run. Best of luck to you in your marathon!!!

Perhaps the issue isn't caused by improper nutrition but by running to fast for your long runs?

Can you define what you mean by bonk? I ask because it's a commonly used term which is greatly misunderstood and misused.

To truly "bonk" means nearly complete glycogen depletion. Your body shuts down your muscles to save what little remains for brain function. You're running along, perhaps tired, perhaps sore, but your running, and suddenly your can't any more. You can barely even walk.

If you're running 14 miles at, say 9 minutes per mile and suddenly your pace drops to 11 or 12 minutes per mile because you feel tired and can't move your legs any faster, that is not "bonking". It's fatigue.

If it's really bonking you can solve the problem by running slower which increases the percentage of fat your body uses for fuel (and trains your body to be more efficient at using fat for fuel), top off your glycogen stores the night before as discussed above, and/or start taking nutrition immediately on your long runs. If you're running marathon pace, your body is using glycogen faster than you can take it in, so you might as well pop those gels from the beginning and start getting those carbs processed. I've read it takes 30 minutes from the time you consume carbs to the time it translates into energy for your muscles.

Since my main running goal is trail ultramarathons, I'll often run my long runs fasted and without fueling during the run to get my body to burn more fat. My long run so far doing this is 19 miles.