Hi. If you use a standard voltage regulator, you will waste a LOT of energy this way, and regulator will get hot and require heatsinking.
I would recommend a module like this http://goo.gl/nLeVz - they are very good at this job.
Please be sure not to over-discharge your battery pack, or you will hurt your batteries. You could make an auto-switch off circuit with some resistors and a transistor that turns off the load when the voltage gets too low.
Good luck with the project.
// Per.