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.
I too am looking at my first Ultra. I have previously run marathons one of which being the Neolithic Marathon.
I am looking at The Ox (white star running) which is 35 miles and seems to fall on the May bank holiday and I can second The Salisbury 54321 I have done the 30k and aside from the lack of signage around the last bit in the city centre its a good set-up.