For all my marathon buildups (8 of them now), I've alternated long run workouts one week with easy-but-longer long runs the next.
For half the buildups, I did the easy long runs not just without calories during the run, but no calories before the run either (I did the runs in the morning, so the last calories I had before these runs were the night before). I would hydrate, but with water only. During the workout long runs, I would fuel beforehand and then use what would be on course to fuel during the workout.
For the other half of the buildups, I fueled before and during all long runs, including with the drinks and gels that would be on course.
The outcome: The races for which I did the "no fuel" easy long runs all went well and I never felt like I was bonking in terms of energy (I was still feeling it muscularly, of course, but never had the "out of fuel" feeling). The races for which I fueled during all long runs did not go as well, including a couple of definite bonks (second half 10 minutes slower than the first for a 2:30-ish marathon, for example).
I'm just a sample size of one, but I seemed to handle the "no-fuel" runs just fine in training, and seemed to race better with them. I would definitely recommend trying it for a training cycle to see how it works for you.