Answer:
My life can be measured in hours, I serve by being devoured. Thin, I am quick. Fat, I am slow. Wind is my foe.
Goes over all the hills and hollows. Bites hard, but never swallows.
Two horses, swiftest traveling, harnessed in a pair, and grazing ever in places. Distant from them.
You seek it out, when your hunger is ripe. It sits on four legs, and smokes a pipe.
The more you take, the more you leave behind.
I am your mother's brother's only brother in law. Who am I?