I did 250 miles over 3 days on my fixie last year. Bike has bosses for two bottles, and I also clamped a third under the down tube with an old steel cage clamp designed for such things. Seat tube is long enough that I could actually clamp another cage above the existing one...would make it hard to get out, but I'm more concerned with just having the water than having it easy to reach...also, I hate the feel of anything on my back during long rides. I sweat like a pig: need lots of water, need no clingy shirt or pack.