Thanks for all the responses (and am amused to see someone even has the user name sorequads!)
Philomena Cunk is correct, it was Brighton Half. So no real downhills to explain it. Is interesting to see others who ran in Brighton also had sore legs/quads. Probably was the repetitive running on the flat/pushing myself. I ran a 3 min pb so we going for it! Legs not so bad today thankfully.... yesterday stairs involved a lot of saying "ow!"
Not concerned as I know it's DOMS but was thinking about leg soreness and wondered if anyone has any insight. In training my quads never hurt, I generally give them a cursory post run stretch and then ignore them. I ran a fast (for me) half marathon on Sunday and today my quads are the only part of my legs that are sore. So is this likely to be a sign they are a bit weak? Or a sign that faster running uses my leg muscles differently? Or just that they're a big muscle so did a lot of work.
If I were you I'd set out trying to keep somewhere between 9 and 9:30 mm pace. If feeling good on the day try to keep to the lower end of that band. If you fade a bit in the second half you should still pb.
Your training paces sound pretty similar to mine, and this advice is basically what I've done on my last 4 Halfs. It got me 2:03, 2:03, 2:02 and finally after 12 attempts, 1:59!