I found that using the progression run approach outlined in Tanser's book really helped me get in some "longer" am runs while at the same time preventing me from getting those too-fast-too-soon injuries. I used a HRM (as always) and started at an absolute crawl (like 50%) and increased the tempo adding 1 bpm to my HR every 30 seconds. So, I would start at like 100 (which is basically walking for me) and by 30 minutes into the run be hitting 80%. Typically, I wouldn't cover much distance for the first 5 minutes, then, by 10, would be feeling okay anyway and gradually built steam this way. If I felt the pace was getting too quick, I could just stop increasing the intensity at a given HR/pace. The hardest part about this is moving easily enough to keep your HR from jumping up 5-10 bpm when you start.