I have non-running friends who last October were puffing after half a mile. They then got to a point they were grumbling at running "12 miles this weekend" and by the end of it they were cheerily saying "ONLY 12 miles this weekend" They got around London in just under five and a half hours.
Totally agree speak to your GP, preferably your neurologist as NEADs and epilepsy can be so, so different from one person to the next. I can only really comment on the seizure side as I believe the causes for NEADs can be different from that of epilepsy although the outcomes are the same.
Is there a pattern to your seizures? Do you know what causes them? Are you suffering from full-on tonic-clonic seizures or "funny turns" (excuse the term funny turns but they can be so varied from staring into space to walking around in circles mumbling I can't think of another word.)
There are a few devices out there that can sense and send an alarm to a friend/relative if they sense you are having a fit (I think they are based on if they think you have fallen over).
If you decide to run always let somebody know where you are going and how long you expect to be. I usually have a piece of ID on me should I end up in a hedge or something.
Definitely echo Cougie. Little things like a small breeze, a slight camber in the road or temperature will make a difference. Just like you have been training yourself to run your body has to get used to be outside.
You will have runs where you feel you could conquer the world and runs where you feel like you are running through treacle. It happens to everyone be it an experienced marathon runner or a first-time 5k-er. That's just the way it is. But you are out there, still doing it so high-five to that!
Have a go at a sports massage just to loosen it off as a tight ITB will often present with knee pain. Once it is loose you will be able to go and do the yoga etc as Cal says from a zero point as it were.