There are a few different threads here devoted to this topic you might want to poke around. My impression is that people with blood sugar problems (who crash and burn when eating carbs) tend to perform better in endurance events when they go lower carb. Other folks like me with no insulin resistance (got told yesterday I will never develop Type II diabetes with my perfect blood sugar levels) tend to perform better with the extra carbs. So, one thing you might do is try to figure out what type you are. If you are sensitive to carbs, you might be able to adapt to running and low carb.

My husband, for example, does great with running on zero carb (he's run up to about 8 miles on zero carb and raced 5 miles), but I would really bonk. You can train low carb and just use gels and whatnot for races, but for longer distance runs that will hamper your training and recovery so it's a balance. Last summer, every weekend practically I was running between 13 and 17 miles for my long run and I would not have survived those without carbs. Another tactic is to use "better carbs" and fewer of them - no gels or blocs, just real food and drink something like coconut water. When I do long runs in the summer (12+ miles) I drink coconut water and take a baggie of sea salt to eat from. Some people make their own sports drinks with honey and lime juice. Some people eat chia seeds.

Good luck! Which marathon are you doing?