The race is over and I'm still here - Result! Quick race report:
Apparently there were upwards of 3,500 people running, so once I had walked across the start line it wasn't easy to run in a straight line or at a steady pace for the first kilometer and a half-ish. After that, I just settled down to a steady-ish pace for quite a while, which seemed faster than other people's steady paces as I kept making progress past people up 'til about 5K. After that, it got pretty tough over the Cardiff Barrage to about the 7K mark, and I wasn't passing anyone any more. But by the second water station after 7K, I knew it was under 3000m to go and that I had the energy to do that distance at a decent pace, so I started going a bit faster again and closed in on the finish, getting a few more places en route. I wasn't sure what the last K was like so I sprinted from a bit far out, but still got there
Gun time was about 43:30, can't confirm that as water must have got in the mechanism of my watch.