I too just completed 3 weeks on Keto. I am a runner and I know exactly what you mean about your legs feeling heavy. They start to go to jelly for me now after about a mile.
I have been told this is normal. It’s so hard to back off, but I’m determined to give Keto my all, so I’m going to have to.
I did read something very promising by a Keto runner. She said eventually the endurance will come back and she now runs 2 hours at a time before eating with no problems. As I’m trying to go for distance, that sounds great to me. She said it would be about 6 weeks before energy levels started to come back. But my energy levels sound like they may be even lower than yours. I’ve actually had days where it’s difficult to get out of bed and do anything.
I made the huge mistake of signing up for a RunBet challenge the same time I started Keto. Every run feels like it’s gotten worse than the one before. But I only have one more run to complete. Oh, and my husband was so sweet as to sign me up for a 5k for mother’s day. Something I really wanted before starting Keto. I don’t have the heart to let him down, so I’m I’m going to give it my all.