The article linked was sent to me the this weekend and I think the idea behind it is dead on. I can’t verify the 40% percentage, but the general point that your body can do so much more than you think it can is dead on.
Right now, this very moment, no matter what your fitness level is you have a certain point at which you feel you have reached your max. It is good to push your body from time to time. The only way to make yourself stronger is to push past what you think you can do. You aren’t going to build bulging biceps by curling a can of green beans 30 times a day. You also aren’t going to set a new PR or finish a marathon if you don’t go harder than you think you can from time to time.
As you train to meet a goal in running most of your runs are OK to be at a conversation pace. These run build your stamina and endurance. These would be most of our Saturday Group Runs. We get out there and run to build a base of mileage that our body learns we can do. Sometimes though you have to go out there and run a shorter distance at a faster than normal pace. Instead of a 5 miles steady pace run 3 miles 1 minute per mile faster than normal. When you get started you will likely feel OK. After mile 2 you may think this is impossible to sustain, but really that is where the 40% rule comes into play.
Your body can do so much more than you let your mind think it can. Get your mind out of the way and buy into the 40% rule mentioned in this article.