I lived in Boulder for about 3 and a half months and I drove to trails 2 times that I can remember. I thought there was plenty of terrain that could be reached from my door. Come to think of it, there was also one place that I would ride my bike to every now and then, about a 10 minute ride.
Some of the trails are a little hard to access just because you have to run up into the hills to get to them but if you come to Boulder to run a lot and run on hills, then that shouldn't matter much.
As for the people, sure there are some elitists, but I'd say that Boulder is a good place to go for a summer with some people who you know you will enjoy spending time with.