Adopt, don't shop. It irks me when people think they need a specific breed for a specific reason. I've seen people get a "running breed" dog who hates to run. Just like people, dogs are individuals and you should think about more than what "use" you can get out of them.
Stepping off my soapbox, I have a 16-pound rescue dog that happens to love running, so I lucked out. I did not think she'd be a runner when I adopted her because she's part chihuahua. The other part, my best guess, is whippet. Whippets are in general good runners. Italian Greyhounds as well, which are similar in size.
I would not run 50 mpw with my dog though. She would if I let her, but I don't want her to get hurt. Sometimes the skin on her paws wears off and I have to let her rest for that to grow back, for example. If the weather is good, I generally take her on most of my 5-8 mile easy runs, so probably around 20-35 mpw.
As far as heat, dogs are very sensitive to it because they do not sweat like we do. Heat is very dangerous to dogs. Not only the air temperature, but the road or sidewalk temps. I don't run with my dog when it's over 70 degrees.