Well, BC, you are experimenting, which is what you need to do! Maybe it was just an off day. Any number of factors can contribute. Hormones, hydration, attitude, alignment of the planets. And for the record, there is nothing wrong with taking walk breaks throughout the run. It probably won't affect your overall time that much and it gives your body a little break. Unless you are racing to win, of course

My hunch is that you might not be taking in enough fuel. What is the timing of your gel? Is the interval more than 40-45 minutes? Every person is different, but as an example, I fuel every 30 minutes (I try for 20 to 30 g of carbohydrates at each fueling) whether I think I need it or not (and I normally start the solid fuel at 1 hour). I cannot remember the last time I bonked and I never feel like I am getting too much fuel (the exception to that being events where I tend to get off because the aid station spacing never matches my regime). Is your gel plus the Perpetuem approaching 55 to 60 g/hour? It's a fine line: too many calories and your body can't process (with the standard reaction being GI distress). Too little and you suffer miles later.

However, given that you felt bad from the start, maybe what was affecting you is what you ate (or did not eat) the day before or the two days before. Are you getting enough low GI carbohydrate (enough to recover in between your long runs)? I've found that can make a big difference. I think that's more important than what you might eat immediately before you go for a run.

Good luck with your 20. It's hard. I don't care what anybody says. I hope it is better than yesterday's run!