Can't believe no one has mentioned the Raleigh/Durham area of NC yet.
- Warm most of the year: check
Summers are hot and muggy, but definitely no worse than some of the other places mentioned here like Nashville, Austin, and Tallahassee. Winters are generally pretty mild - there's usually 1-3 snow "storms" of 1-6 inches that melt in a few days.
- Affordable cost of living: check
NC does have a (modest) state income tax, but you'll find the price of housing, consumer goods, etc. to be quite reasonable
- Decent running community: check
There are a lot of great races, running groups, and places to run in the area. My favorite places are Umstead Park, the American Tobacco Trail, and Duke Forest.
Other perks:
- Raleigh-Durham Airport (RDU) has many connections across the country, and is super-easy to get in and out of.
- There are a lot of young professionals in the area with the Research Triangle Park, grad students at Duke, UNC, and NC State, etc.
-You're only 2-4 hours from both the mountains and the coast, both great places to get away for the weekend.
- Between the Symphony Hall in Raleigh and DPAC (Durham Performing Arts Center), you get the chance to see world-class arts and theater that come through town.
Also if you like the mountains and don't mind a little bit more of a smaller city, check out Asheville. If I could have my same current job in the Triangle instead in Asheville, I'd move there in a heartbeat.
I've lived in both Nashville and the Triangle, and Nashville really doesn't compare when it comes to running and overall quality of life, in my opinion.