Every week when I'm out doing my long run I always feel incredible about 45-50 minutes in, and my legs start flowing in a way I can never match if I just warm up. A lot of times I end up really cooking in the last four-to-six miles. I'm never completely huffing and puffing, but I'm no longer in that hold-a-conversation pace that's supposed to make up the majority of your mileage.
My question is if there is any harm in this type of running. I'd also like to know if you tend to experience the same thing (I've heard a lot of runners talk about this) and how you think it has effected your training.