Unfortunately, training "smart" is not always cut and dry. As athletes, we are always striving to get to "the next level," and to get there it takes hard work, maybe harder work than we have ever done before. So how hard is too hard? For me I have found a few keys that let me know if I am pushing too hard in practice. I feel like it is ok to be tired on certain days, but I should feel that my legs are "responding" well to the training. For example, I always am looking for a little bit of freshness in my legs. I can pretty well tell how my legs are feeling by doing 200s or any type of strides. If I feel a little spring in my step than I know that my legs are absorbing the training well. I can also tell how my legs are doing on my easy runs. I take my easy runs really easy. I always just pay attention to how my legs are feeling and essentially let them dictate the pace. So if I am moving along effortlessly on an easy run than I know my legs are feeling pretty good and ready for another hard session. Perhaps the biggest indicator is seeing improvement in my workout times over substantial periods of time. This doesn't mean that if I repeat a workout I did two weeks earlier and run slower that I am over trained but if I repeat the workout multiple times without seeing any progress or at least feeling better than maybe I need to take it easy for a couple of days.
-Ryan Hall, nyrr.com