Slow run without pain?

Looking for tips on how to run at a slower / conversational pace without calf pain...

Started a running program about two months ago, focused on run / walk intervals and gradually increasing the amount of time spent running. Now, however, I'm getting to the point where I need to slow my pace down in order to not burn out before my next walk interval on the long runs. However, when I slow my pace down my calves start to hurt pretty quickly.

I've got the right kinds of shoes on, and I've focused on shortening my stride, light feet, etc. but nothing helps. If I'm running at about a 9 or 10 minute mile pace my legs feel fine - they fatigue a bit, but they're fine. If I slow down to a 12 or 13 minute mile pace, boom - calves on fire.

Is it just a strengthening issue or is there something else I can try?