These are my Nike Air Zoom Pegasus 31's. They've not done a huge mileage, and must be two years old. But look at that weird wearing, it is a scuff on the inner cuff. Both shoes are the same, but this one is a bit worse than the left shoe.
You know that staggery feeling, when you've just run 10km (or a HM, or M if that's your thing)? You've given it your best, got over the finish line and then your posture goes, and you stagger around a bit. You might blame it on 'tired legs' but your slumped posture is not anything to do with your legs, is it?
No. That's your weak core making you slump after a race. Literally, your body cannot hold you up: your core is exhausted.
It's the same with my trainers. The scuff is caused by my tired body not holding my pelvis straight, and on each step (after about 8km), the sole of my trainer snags against the other shoe. I blamed that on 'tired legs' too, but it is my poor core strength.
Must work on my core!