Two thoughts come to mind, hopefully somebody can expand on them. One is blood pressure, it will drop after a couple minutes of stopping exercising. I'd probably want to know what it is when you feel like you do. Dropping too far could give you a lightheaded feeling. The other is blood sugar dropping. I asked one of staff at the gym this, I jog as part of my routine and if I run too fast I can sometimes feel nauseated, maybe a little lightheaded, why it that? It was suggested that perhaps a blood sugar drop may be part of the answer. Burning glycogen too fast? I think it's possible to eat some candy or perhaps a sugar cube during or right after your walk and see if that helps at all. Again, these are thoughts, something to think about, I could be way off base too!:) Other than that, I don't think one could push themselves by walking too fast?:confused: