If they could talk, your legs, they will tell you to stop. They would complain about the torture you let them go through. They would air their disaprroval of being dragged out of bed in the very early morning when sleeping was the natural and logical thing for a sane person to do. Your lungs would perhaps be joining the chorus too.
After running 15 kilometers, I felt all that. My lungs and legs were not talking, they can't, but they were telling me just what they were telling me. They'd rather not be doing what I had in mind. There were three uphills ahead, an equal number of descents, maybe a couple more. I would do surges through them.
I can feel the exhaustion everytime I picked up the pace. There was no pain, nothing like it, but there was fatigue that made my legs feel heavy. There was no smoothness in the stride. There was labor in each step.
There is nothing easy here. There is nothing to make you feel ecstatic. There is only difficulty and suffering.
But after you have finished that last leg, that last kilometer, with a surge to the end of your run, after you have given your all , after you stand bent at the hips, catching your breath, your two hands resting on both knees, you straighten up and walk slowly, walk tall, proud to have silenced that voice inside that always tells you to stop.
You have conquered. You have won.
p.s. I posted negative splits in those last 5 kilometers of my 20k run and had a faster second half overall. Sweet exhaustion.