On Tuesday of this week I did the greatest tempo run of my entire life. Was cruising for 6 miles at paces that I would normally hold for 1mi/2mi threshold reps on the track. Unfortunately I may have pushed a bit too hard or maybe just got a bit unlucky and sustained a minor overuse injury that I had to take 4 days off to recover from. Today was my first run back, I did a 6 mile run at a pace over 2 minutes per mile slower than my tempo earlier this week - now my lungs are super sore, my legs feel shot, and I'm just generally exhausted. How is it possible that I lost so much fitness after just 4 days off? Is this normal? This isn't my first time experiencing this problem. I know that the fitness will return rather quickly but this just blows my mind.
You didn't in 4 days. You just had a bad day probably thrown off a little bit by your time off
I assume this is a troll, but in case it’s not not, you have a misconception:
Fitness or lack of fitness ==/== fatigue or well rested. It looks like you conflated these.