I ran 19 miles yesterday and I don’t hurt today. I was sore immediately afterwards, of course. As soon as I got home, ate, tossed my clothes in the washer and showered (it may not have been in that order) I napped for a good hour. But once I woke up from my siesta, I was good to go.
The secret is, I’ve learned my pace. I know just how fast I can go…forever. May be not that long, but a long time. I know it by feel and effort, by cadence and heart rate. I can drop below it and push above it, and I understand what will happen in each case (boredom and destruction, respectively).
I recommend finding this pace by feel rather than watch. It’s a moving target but once you know it in your bones, you’ll feel capable of anything.