That does seem short, its specs say it should get 16 hours but that is using their own batterys (not AA) and they always exaggerate
The oregon 600 is very fussy about its power source. If your using alkali batteries see if you can get two NiMH or Lithium AA batteries instead as the manual says they will work better (especially if you plan on using its camera) and they are more suited to outdoor temperatures. You would also get the benifit of being able to charge them. Check that the settings on it have your type of AA battery selected as well.

Also try managing your power use. Have the backlit turn off after a few seconds of no use and turn on "power saving mode" in the settings which will turn off the screen completly when the backlit times out. Try and keep your screen brightness at the minimum you need to clearly see it, instead of having it allways at max. (Not sure how easy it is to change the brightness on it, if its very accessible you can change the brightness while your using it)
You could also decrese the mapping speed as well, depends how detailed you want the gps to recored where you walked.

Enjoy your birthday and new GPS!