Jokeybhoy - My half PB at Dunoon came two days after an 18 mile training run. When I ran my first PB of the year at the Loch Katrine half I'd ran a 20 mile training run three days before.
I don't know if its just luck or I'm doing something right, but I recover very quickly from long training runs as long as I've kept an easy pace during them.
I think I also benefit from being an idiot and having a get yer legs shiftin' ya big eejit mentality as soon as that start gun goes. I don't clock watch, check HR, or anything else during a race of that distance. I ran three halfs this year and ran three PB's. Not once in any of those races did I check my pace or look at my watch to see what sort of time I was on for. The only on of the three halfs I had an idea of what time I would be around was Dundee, and that was because I knew what sort of time I should be posting over that distance and I knew I'd been pushing hard during the run.
The support along the course is busy at points, quieter at others. For the most part it's fairly busy, but if you look at a map of the course you'll see there are two parks you run through and the support is noticeably thinner in those areas.
Out of breath early on could be due to starting from cold. There can be a lot of hanging around before a race, which isn't something you'd be doing before a training run. Also that hill at the start of the Glasgow Half is good at knocking the wind out you before you've really got warmed up.
I'm certain the first mile of that race is my slowest of the race every time due to the hill before the legs have really got going. I've found I'm normally about mile two in that one before I settle into a rhythm.