Very helpful answers, thankyou everybody, especially Jasmine again.
Interesting to hear about drinking. When I got up to about 11 mile runs, I started to feel that I could do with a drink. It wasn't absolutely necessary but would be nice. Got up to 13.5 miles yesterday, half of it on hilly trail, the rest on flattish trail, and was glad to have a drink.
As a side note, I started off with a bit of a stiff/sore knee, but it seemed to settle down, and I ran with no issues. Woke up with a really sore knee to the point of having to hobble downstairs in some pain. Had a little think about what it might be and tried a deep quad stretch. Instant relief. I reckon my quads had tightened and were pulling my knee cap off its track. Must do some VMO exercises and stretching regularly now, I think.