I use apostrophes as it's much more than a race pack. This would be a suitable pack for most any long day adventures. AK is not for Alaska :), but Anton Kupricka of ultra running fame who helped design the pack.
The first thing that is obvious (and unique) is it's made to carry two water bottles (20 oz variety) on your chest. I prefer bottles over bladders for several reasons, foremost is I keep one filled w/ Perpetuem, which in the past meant carrying a handheld in addition to a pack; I also like to know where I sit w/ my water supply- hard to do w/ a bladder; bottles are much easier to fill out of streams (or anything for that matter) and easier to purify as I use a Steripen
The pack is light, 6.0 oz (6.1 on my scale for a M/L) and utilizes a variety of mesh and even some cuben. The bottles are very easy to access (and replace) on the fly. The stretch mesh secures them in addition to a small bungee/cordlock setup. Below the bottles are two small mesh stretch pockets- sized for electrolytes, blocks and the like, they are secured w/ velcro flaps . Above each bottle are two more pockets, also stretch mesh- sized to fit a smart phone, I put my small camera in one- it's a little tight, but works.
It utilizes two sternum straps, the upper is elastic to allow movement of the chest. Both straps are able to slide up or down a neat little rail system so you can fine tune the fit.
The rear of the pack has two small zippered side pockets, I carry my phone/keys in one and some emergency bits (including a headlamp) in the other. These pockets cannot be reached on the fly (unless you are Houdini!). The main pack pocket is a stretch mesh w/ a outer bungee setup that serves both to compress the load or secure additional items (rain jacket comes to mind). It's not overly large, but because it stretches it holds more than meets the eye- I had a windshirt, emergency blanket, buff, beanie, gloves and extra food and there was still plenty of room. Yesterday I rolled up a R1 vest to take w/ and it fit w/o any problem.
the main pack pocket w/ hold a smaller bladder if you need one, it has a velcro loop to hang one as well as several loops on the outer perimeter of the pack to route a hose. A small bladder would obviously consume some of the available volume.
Well this is all well and good, but if it doesn't carry well when running it's not worth a tinker's dam n, I'm here to tell you this little pack (w/ full bottles) carries like an absolute dream :) This is really well thought out little pack that works>