http://runningtimes.com/Article.aspx?ArticleID=6765
If you go to that site, it shows what you should be able to accomplish at certain distances if you can run a certain time at another distance. I put in 50 minutes for 10km, and if you can run that, you should be able to run a half marathon in about 1:50. That's if you're in top shape though and can handle the extra mileage.