I've been noticing when I get super tired on a run my feet start shuffling along the ground. It makes it so hard to pick those feet back up if I've been shuffling for a while so it's hard to pick up the pace. I focused on the Saturday run on really lifting the legs a little when they would start to feel tired. Even motivated myself out loud with "Stop shuffling, stop shuffling".
Have you paid attention to the sound of your feet on the pavement lately? Or can you even hear it over the music on your iPod? Something to think about... Some aches, tweaks, and pains and things affecting our speed and running performance can be discovered if we listen to our bodies more!