1: Not every ‘workout’ leaves you totally exhausted.
2. Unless you’re doubling every day, you are recovering for 24 hours before your run anyways.
3. In a typical “moderate” run you usually start fairly easy and naturally pick it up. But very little, if any, of the run is faster than threshold. It’s not like you’re actually pushing hard. *Note, if you are a beginner/amateur runner chances are you actually are running faster than your threshold, so slow down lol.
4. There’s more to running than just energy systems. Running economy is a pretty big factor. Running closer to race pace allows you to run at a pace biomechanically closer to how you run at race pace, which can improve running economy.
5. Some People actually make improvements by running their non-workout days a little faster due to some higher-end aerobic work. (What, no way, I don’t believe it... no running book says that. If you run too fast on easy days you will instantaneously be overtrained and never PR again... in fact you’ll burn out and quit running) Lol, in reality, I’ve seen many people increase the pace of their non-workout days and improve. But not every day was moderate, some days were still slow, the key was the effort was about the same. The focus should be the workouts and you should always run easy enough that you feel ready to hammer on workout day. But if you feel fine running a little faster on ‘non-workout days’ you may actually get more fit- just don’t push to run harder... it has to come naturally and the effort should stay fairly low.
6. People recover at different rates. Some people NEED an easy run after every workout and their body feels it. Some people bounce back super quick. That’s just how it is. So you really can’t give concrete answers that apply equally to everybody.
7. Running can’t be a mentally numbing and monotonous sport. Only a select few can be crazy rigid with their paces every day. Give yourself some freedom to just run and forget what everybody (even science) says from time to time. Don’t make a habit of training stupidly... but not every day has to be textbook perfect.