How small are you?

I've raved at length in other posts about my Nathan Intensity and Synergy vests. It sounds like I'm shaped similar to you - very short waisted and large hipped. But at 5'3" and 120# I'm stocky, not petite. I have the straps adjusted all the way tight on my Synergy, and only a little bit of room to tighten them on the Intensity.

The Intensity only holds two liters, so for marathon training you'd need to be able to refill it once on your long runs, but if you're bigger than about 110#, I'd give it a try.

Neither of these vests is anything like wearing a pack. They really wear like vests.