I'm not sure coming to a downhill, high-altitude race will help you much when you train on flat at sea level. They pretty much cancel each other out, and the downhill can wreck your quads if you don't train on downhills. Not saying it won't help, just that it's not a sure thing.
As for how to knock time off- consistently run as many miles as you can. 6 days a week. Sasha has posted lots of other advice like this. Take a look at many posts here:
http://fastrunningblog.com/forum/index.php/board,3.0.htmlGood luck!