Quote Originally Posted by MCAP View Post
Yes, my running shoes were purchased at a running store after a professional gait analysis.

7.0 is only a little faster than a ten minute mile- certainly not a sprint. I feel very comfortable at this speed but feel like my body may just crumple out from underneath me after about a minute. My legs feel fine, my lungs feel fine, my head feels fine, but my heart races and my body gets weak.

I'm going to try to find a doctor to see about this. I'm very perplexed that it only happens running and not cycling or swimming. Perhaps it has something to do with running being weight bearing.
7mph is an 8:30-ish min mi. An 8:30 min mi is significantly faster than a 10 min mile The likely case, as Jess suggests, is that that is closer to your interval pace than your regular "forever" running pace. What you describe sounds like an interval to me (well, a little beyond - Knot's right that feeling like you're going to pass out is pushing it) - it's hard work for a short period of time. You'll probably get better at them the more you do them, but you will probably not be able to run that speed for sustained periods for a while. If you want to run longer, you will probably have to run slower until your ability to run at those speeds increases. The intervals WILL help with that, so if you do introduce days with more extended slower running periods, you should keep up some faster intervals on other days, too (but maybe not to the point where you're lightheaded ).

I can definitely understand that your stride (and everything, really) is more comfortable at the faster pace. It does force you to stretch and land your feet differently and I also find this difficult to apply at lower speeds. I'm not sure there's a secret to that other than to improve your form - it might do your faster running speeds good to improve at slower speeds.