That said, I can sure make some suggestions to start you in the right direction. I'm a fan of the Ultimate Direction packs; they're light, well-made, and come in several sizes for different activities. For you, their Wasp ($70; www.ultimatedirection.com) might be a good choice. It's designed for trail running and endurance races, and has a snug fit that won't jiggle around when you're on the move. It's got a pretty large bladder with 115 cubic inches of capacity (nearly half a gallon) and just enough room for a few energy bars, a light jacket, and some gloves.
For the same price, CamelBak's Rally ($70; www.camelbak.com) has a similar-sized bladder but more room for gearabout 1,100 cubic inches, versus less than half that for the Wasp. So you can carry more stuff for your longer hikes. The downside: The Rally, even though it's designed with adventure racers in mind, isn't going to fit quite as snugly as the more vest-like Wasp. But it has bungee cords to make sure the pack contents are stable, and with a light load should be fine for trail runs.
One other good candidate is Gregory's Inertia ($99; www.gregorypacks.com). About the same size as the Rally, but of course Gregory is famous for its excellent suspension systems, so it should carry exceptionally well. Worth a look.
Check out Outside's reviews of the best hydration packs in the 2004 Buyer's Guide.