I'm back in Cape Town, so here's the final report. The official time stands at 3:57.34, with fairly even splits. I'm very happy to be inside 4 hours, and trying to go a bit faster is probably the only reason I'd do this again.
The event is an amazing feat of logistics, with 36,000 runners and thousands of spectators lining the route. There were jazz bands, rock bands, people handing out food to complete strangers, it was amazing. Having your name on your shirt helps as much as everyone says - having people cheer you by name all the way round is great. My faithful supporters were at the Red Cross cheering point at about 25 miles, which was something to look forward to. They gleefully told me that while waiting for me to go past they saw a guy with one leg, a bloke dressed as a banana, and a couple of fairies. They also say I looked quite strong at that point, but you can judge for yourself from the pictures below.
Many thanks to everyone who turned out, sponsored me, sent me their good wishes, and listened to all my tedious running chat for the last 6 months.