from what I understand, being relatively new myself, your body is still transitioning from glucose to ketone fuel. right now your body is still trying to find its energy from glucose, so to go out running you're going to feel a bit sluggish and fatigued.

I had the same thing happen today.. I went out for a run and I felt like I had just run a half marathon yesterday. I was beat after about half mile.

From everything I'm reading it gets much better.

If anyone else has more info that would be great. I looked but I'm not finding the article I read that gave me this info. In a nutshell, your body is going through reprogramming, and four days isn't enough time for it to go out running and to not feel a big difference.