The first is the No Meat Athlete. Even though it's geared toward vegetarian endurance athletes, all of the columns I have read have had personal relevance. It's witty, honest, and thought-provoking writing. A few favorite entries are 27 Things Your Training Partner Won't Tell You and My Big Screw-Up: Why (and How) I'm Training for a 50-Miler in Only 4 Weeks.
The second one on my must-read list is Endurance Corner. The contributors include coaches, elite competitors, sport psychologists, and nutritionists. I find much sage advice here!