Resting is when your body adapts, recovers and rebuilds to make it stronger. This will only happen if you stress your body in accordance with the adaptation you want to happen (e.g. if you want to get faster, you need to practice running faster) and you let your body make those tweaks.
That’s why a long-run schedule takes backwards steps sometimes. It can be tempting to keep going up, especially after a good run, but don’t stress your body too much, or you’ll end up over-training and sick or injured.
This article from BrianMac Sports Coach explains it beautifully.