I learnt two things in my run this morning. One: iPhone GPS doesn't seem to work when it is misty/drizzly/cloudy. This sort of makes sense to me; our satellite TV reception in the UK used to go to shit when it rained. Of course, I could be wrong. More experiments are needed. Anyway, this meant I couldn't upload my run data to trailguru and get some nice graphs.
Two: running faster makes me slower. I was trying to speed up a little on this run, doing the same distance as I ran on Monday, but just a little bit faster. I must have gone a bit too fast, because I had to stop and walk after about 9 minutes, something I haven't had to do for a few weeks. To improve my speed I think I need to monitor my pace via the trailguru thing as I'm running, and make sure that I'm only increasing it slightly. Altogether this run took me about a minute longer than Monday, although without the GPS data I can't really tell how much faster I was running in the first half or how much slower it made me in the second half.
UPDATE: I don't think it was the mist. The 2.2 firmware update borked the GPS temporarily, I think. A reboot (hold down home and sleep) sorted it out.