A steady headwind is blowing down the 17th major hill you've tackled today - the last town was hardly a stop sign, and the only open store was a dilapidated Exxon station (they still had the old "Esso" sign rusting in the weed choked side-yard).
So you bought the out-of-date snickers bar, and a pint of Lipton Iced Tea... but you're BONKED... even with the 4500 calories you've scarfed today. And the campground is still 10+ of straight up and down.
