When I had a regular running route, I marked my distance by familiar landmarks - the farm where the cows never came near the road, the house where the deer would often congregate along the edge of the woods, the pothole that I somehow managed to thankfully avoid.
Writers don't have concrete landmarks. We mark our pace when writing by our own sets of rules - the first chapter and perhaps if you write like me - the last chapter at nearly the same time, writing "The End" even when it isn't...
Today I hit the 50-page mark on my latest project. Like the hopeful millionaire watching the balls spin in the mesh tube, I watched the page count roll over to the "50" mark and sat back with relief. It doesn't seem like much, but to me it's a milestone - a landmark that says "I'm well on my way."