Sometimes a week isn't enough.
Maybe you are fighting off an infection, tests are unlikely to show that up unless you monitor heart rate on a regular basis.
I quite often find that when I'm struggling with longer runs then spending time doing really short stuff helps. Today I ran reps of 100, 200 + 300m. Yesterday I could barely jog a mile but today I really pushed as hard as I could and found that I had the energy to do so.
I don't eat before running but have been known to knock back a coke or red bull esp if I'm tired but need to get home or to work.
How are you sleeping?
Any stress in your life?
Have you raced recently or pushed yourself hard in some way?
Maybe vary your runs. Do a fartlek session for example. You could just be bored.