1h39m sounds like a pretty good time!
I was running with (or rather started with!) a couple of friends who run for their university and they were going for 1h30m. They ended up getting 1h50m, which made me feel better about using the heat as an excuse!
I would love to do another 1/2mar on a cooler day and will aim for 2hrs.
hilbert - I had three weeks notice to train and must have done a few half hour runs and two hour long runs. My first hour run felt painful, the next one was fine (even fun!). I plan to keep running and just increase my longest runs by 15minute increments. That way I will soon find a 1/2 marathon comfortable and then maybe a full one some day.
I also learnt the importance of wearing the right shoes!
EDIT:> Just got my placing: 892 out of 1015

. Not exactly spectacular, but at least there's lots of room for improvement