I get mid back pain while running. it aggravates when i run more. searching on the web tells me it's a posture issue. what can i do to alleviate it?

May be: It may be bad posture and wrong technique. Also r/o exercise induces reactive airways. Talk to your md.

Adjust your stride: It could certainly be related to your running form. Try modifying your stride so that you strike the ground with your mid-foot rather than you heel. This will promote a better overall alignment of your spine while running.