Part I - Background
There are lots of discussions out there about people doing endurance exercise on "low carb". I put that in quotes, because usually the advice is to "train low, race high", carb-load on sweet potatoes before the race, and so forth. Mark Sisson's recommendations fall into this category.
But, there are very few reports of people racing on *very* low carb, i.e. in a state of ketosis. So, I thought I'd provide one such report. First, some short background. I got the idea to try this from reading the book Low Carb Performance by Volek and Phinney. It is a really interesting book for anyone interested in this subject. They recommend consuming less than 50g of carbs per day to get into ketosis, and they argue that it's not only possible but beneficial in some ways to train and race in this state indefinitely. The idea is that in ketosis, your liver produces ketones, which can provide up to about 3/4 of the fuel that your brain needs, instead of your brain running on glucose only. This is good not only because is spares glucose, but also because you won't run out of brain fuel and "hit the wall" -- the liver create ketones out of fat, so there is a virtually unlimited supply.
In addition, your muscles adapt dramatically to burning much more fat and much less glucose. This is beneficial for endurance exercise because your stores of glucose are no more than 2000 calories or so, whereas your fat stores are, for practical purposes, unlimited. Some other purported benefits of ketosis are faster recovery, less oxidative stresss and inflammation, being able to go for a long time without eating, with even energy levels and blood glucose, and easy weight maintenance.
The only similar reports I've found are:
Peter Attia's blog
Paul and Cynthia's report on MDA
Some background on me. I'm 34 years old. I have been doing the primal lifestyle for almost two years and love it. Trail running is my sport of choice, but I don't take it too seriously, and don't run a whole lot in training. My exercise for a good week looks something like this:
* bodyweight exercises 1-2x
* rock climbing 1x
* swimming 1x
* hiking 1x
* running 1x
Certainly no "chronic cardio" going on here.
The marathon in question was the Coastal Trail Runs Crystal Springs Trail Marathon. It was to be my first marathon.