newbie, exhausted AFTER ride?

[KnottedYet]

Sounds like you put out a lot of effort and you were tired. Your body "pushed" those 13 miles after you decided your regular pace was too easy, and then said "OK, time for me to build muscle because the muscles sent the fatigue signal, so now you take a nap so I can do that."

We build muscle when we rest. Effort and exercise set up the signals to the muscle-building system. At the next restful opportunity the muscles build. (Like during your 4-day break.)

I bet when you do that 20 miles at a "not pushed" level of effort, you will find that they are a "piece of cake", too!

Knot-who-just-woke-up-from-a-nap-after-riding-45-miles.