Many people suffer from either pushing themselves too much or not pushing themselves enough. Seems obvious enough, right? The problem really lies in WHEN they push, or don't.
Too often people get stuck in a pace where they are working to get comfortable at a pace that is tough. They then get acclimated to that pace, it becomes easy, and they stay there - never pushing harder on their "hard" days or during their speed work. They continue along at a 120 heart rate never allowing their muscles (heart) to grow through strain.
The other side of that is that far too many people don't take their easy days... easily enough. Pushing too hard (since it's a short distance) or, again, running at that same stuck in pace... just not as far.
Case and point: today I drove to work, put my keys on my desk in the office, locked my truck, and took a spare key and stuck it in my zipper iPod pocket on my jacket.
I took off, ran my short run on my "easy" day and ran it... easy? no. Hard. I ran at race pace (what you are NOT supposed to do). I got back, unzipped my pocket, took out my iPod, and... no key. I took the jacket off, shook it out, walked around the truck, walked around the office door, hoping somehow this key would magically appear (I put it in a Zipper pocket with my iPod... the zipper was Still Zipped, my iPod was still there, there's no hole, and... no key... grrrr!)
So... I took out on the course Again, and ran the route, again, looking for this stupid key. Which, of course, I did not find. However, having to run the course a second time reminded me that easy days are supposed be Easy days - take them easy, run easy... you know... like at the pace I ran my SECOND lap...