The Art and Science of Low Carbohydrate Performance by Phinny and Volek is a very good book for runners who want a more scientific understanding of the high fat/low carb approach. Here are some key points.
1. Over many years, our bodies have adapted to burning mostly carbs and storing fats.
2. This can be changed, but it takes 2-3 weeks of severe carbohydrate restriction to train the cells to burn fat as a primary fuel. During this period, performance will suffer and you will feel bad. The reason is technical, but basically, you are starving your brain.
3. The high fat/low carb diet is a no-brainer for the obese looking to lose weight and ultra athletes, but not so much for everyone else.
4. Athletes who don't do ultra events will need to modify their diets to include more carbs/protein after training and before competitions. How much modification? You have to test for yourself to find out.
5. If you cheat on the diet for more than a few days, the cells revert to burning carbs and you have to go through the 2-3 week adaption phase again.
There is also a very wide response to burning fat as primary fuel. Some runners can burn 30g/hour of fats. Others can burn 60g/hour. Runners who fall into the higher range are well suited to training and racing on a high fat/low carb diet.
TL;DR
What works for me was a 2-week extreme reduction of carbs and increased intake of healthy fats. Now, I fast about 16-18hrs about 5 days a week, consuming only healthy fats from 7PM to 2PM. Since I work out in the morning, my body only has fat calories to burn. From 2-7PM, I eat normally... no junk food, of course. Intermittent fasting keeps my cells in a fat burning mode.
This diet has greatly reduced my problems with extreme hypoglycemia, an inherited condition in my family, and it has reduced my cravings for food.
I do not consider it to be a performance diet. For me, it manages hypoglycemia.