I've spent the last year getting into enough shape to be able to run. I'm not really fast or anything but I cover about 6 to 8 miles almost every other day if not everyday. I like running outdoors on trails with minimalist shoes (vibram these days). I feel like I'm reaching a plateau of what I can actually do on a regular basis. I've bumped my run up to 12 miles a few times and I suffer for it the next day. I find myself having to stretch a lot to prevent shin splints and painfully tight calf muscles. I know that the shoes and terrain make it harder on me.

Just wondering if anyone has any tips or tricks to help me push past this ceiling I'm hitting.

Am I expecting to much here to think I can run more than I already am on a consistent basis?

I'd really like to be one of those guys who can run an ultra-marathon but I just don't think I'll able to push myself that far. Are there tricks I don't know about?