The Steripen will treat only one liter at a time (or a half liter, but that won't help you much). Each liter will take a minute and a half to treat. Multiply that by however many liters you plan to take with you, and that's the minimal amount of time you'll spend actively treating your water.
As a Steripen user, in your situation I might decide to go with a gravity filter enhanced by a chemical treatment. I'd probably use some sort of hydration bladder (likely Platypus) to carry the water in.
Whatever you decide, be prepared for the weight - "A pint (half liter) is a pound, the world around." Two pounds per quart/liter, eight pounds per gallon. Be sure to have a pack that can deal with the weight.
Any chance you can cache a water supply half way through?