Have you considered using Couch-to-5K? (http://www.coolrunning.com/engine/2/2_3/181.shtml)
Clearly, if you have to stop after a half-mile or so because your "legs are burning and/or feel like lead", you are clearly running (much) too fast. Being "out of breath on subsequent intervals" is another sign of running too fast. You have to learn to slow down. By that I mean maybe 11:45 - 12:00 miles instead of 11:00. The only reason I can think for your legs to tighten and/or cramp at a slower pace is that you aren't running relaxed. There is really no good reason for cramping at a slower pace. Teaching yourself to relax might be the best first step.