Testing endurance limits

During my run today the thought popped into my head to see how far I could go at my 6:00/km comfort pace.
Kilometer 12 is where I hit the wall.
I was still feeling pretty good at kilometer 10, and thinking I could maybe make it to 15km.
Legs were starting to get a little tired at kilometer 11, but still felt like 15km was within reach.
Halfway into kilometer 12, my legs turned into lead weights.
Fortunately it was on the stretch heading back towards the house, so I was able to finish the 12km run without having to walk too far to get back home.
Pretty good long run today. A little damp from some light sprinkles, but fortunately the rain held off until after I got back home.