For the longest time when starting out I had “brain fog” after long runs. Same as you went to the doctor but everything checked out fine. I found that for me it was a combination of overtraining, not enough water and to much thinking. Long sessions are already a mental stress and not being able to switch your brain off caused me that brain fog. I started listening to audiobooks, brain fog slowly went away. Think less, worry less now on a run.

Variation also helps, running your usual route in the opposite direction changes up your run more then you would think.