We don't recommend you hike 10 miles (16 kilometers) with a pack on your back in any boots -- even the most perfect boots, gifted to you on high by a choir of footwear angles -- without breaking them in first. The need to break in a boot is especially true with burlier boots -- the stiffer your boot, the longer it takes to break in.
The creases you make in your boots as you break them in will form the shape of the boot for its life, so be sure you do it right [source: REI.com]. Wear them around the house with the socks you'll hike in and make sure the lacing is tight against the boot's tongue, which should lay flat. Then start with short day hikes and slowly, slowly increase the distance.
If your new leather boots are killing you and you don't want to buy a new pair, try this fix: Soak the boots in warm water before wearing them with your hiking socks. A wet foot in a wet boot is no fun to begin with and will quickly create blisters, but molding a wet boot to your foot can be a last-resort break-in trick.