I find Houston a fine place to run, in spite of the heat. Smog is not a problem, especially if you run in the parks or in the grassy fields. Of course, if you like running down the roads, alongside cars, then you will encounter ridiculous emissions and diesel fumes.
Two places filled with runners: the loop surrounding Rice University and Memorial Park. If you're looking for hills, then Allen Parkway is a solid spot.
As a college runner raised in Houston (and here until I leave for work), I'm still able to log 80-85 mile weeks in the summer without using the treadmill. I do a ton of doubles, but that's not necessary if you can wake up at 6 in the morning and bang out your 10 or 12 in the morning before the humidity assaults you around 8:30.