It could be your shoes as well. I've been running for a couple years, and take frequent breaks now.

Vary your run; do different distances, speeds etc. But don't neglect shoes; indulge in a new one every 500 Km.

I don't use shoes at all when I run.

However, I would be willing to buy shoes like the vibram 5 fingers to protect me from the occasional pebbles and the dangerous glasses but not modify how I run barefoot.

what do you do in winter?

Run faster.

I just did in the cold. The heat that your body accumulated over exercise really does make everything warm.

If by 'pain' you're referring to the bone and joint pain alot of runners eventually incur, try swimming an/or biking too. I find you can get the same cardio workout minus the pounding and wear & tear.

Mostly, it's my lungs that's getting tired out. My legs are completely fine for a while with some conditioning until I run longer than I used to. That's when I experience pain, because my legs is getting too tired. These days, it's alway one of the other but never both.

At the start, both my lungs and my legs are pretty much exhausted. Now my lungs and legs can endure for much longer.

Same here. People don't know how high one can get just with running.