Overtrain a couple days a week and undertrain the others. This causes adaptation. If you overtrain too many days in a certain week then the next week you will need to undertrain to get the adaption. If you do not then the stress/fatigue builds up until something bad happens-injury, sickness, cronic fatigue etc.
So a good week would be OUOUUOU with the O's meaning overtraining sessions (workouts) and the U's being easy, recovery days. Feeling tired when training hard is normal. You are probably doing oOoOoOo, meaning you are overtraining a lot on your hard days and overtraining slightly on the days that are meant to be easy. There is no point in overtraining slightly-it just adds fatigue with no real stimulus. This also means that what would normally be a moderate workout becomes a difficult workout due to accumulated fatigue. This could be a special session once every training cycle-two back to back workouts on consecutive days or the same day in doubles.
Examples
Hill workout/Sprint session-sprinting with muscle fatigue; the hills break down the muscles/provide stimulus then the sprint session 'wires' the nervous/muscular system to build up in a way not just appliable to hills but to flat ground
Long run/sprint session-Makes running fast more oxidative in nature
Weights (core work)/Long run- weights push core to failure then long run makes it relevent to running-specific strength and specific endurance
Plyometrics/Hills-Plyos trigger nervous system then hills stress the muscles as well
Long Run/Drills-Long run breaks the muscles down then the drills teach good mechanics when tired.
of course, there are hundreds of options....Hills/Sprints/Long Run/Plyos/Tempos/Core work/leg work/drills
But these should only be once a week at most, ideally once every 10 days. Doing this week after week is what causes burnout. Of course, it is possible that the body will adapt to a training load with oooooooooooooooooooooooo type training, but easy/hard is a much quicker and much more enjoyable way of training.
Noakes is the best at describing overtraining.
http://books.google.ca/books?id=wAa9qq9kbncC&printsec=frontcover&dq=noakes&sig=y9zsQ1ttCLh52ipyzGl1uMXydoU#PPA93,M1