Not to be indelicate, but what do you need the glasses for when slogging up a hill at 4mph? Take 'em off. My sunglasses are prescription, and when heading up a humid hillclimb, the glasses go in the helmet vents-- that is if the helmet hasn't come off already. I've little need for eye and crash protection when moving slightly faster than walking speed.

I will also recommend Wiley-X. They have a whole line of motorcycling sunglasses with removable gaskets so they seal around the eyes, and pretty much all of them are available with prescription lenses.