Two comments. First my sister lives year round up in the Mt Charleston community. She is not a backpacker, but she hikes the trails every day (snowshoes in the winter). I made it up there a year ago: beautiful place. I wish I had the potential to backpack from my front door.
Second, I am currently training at the gym, prepping for an AT section hike this fall. My work schedule has dictated my training routine. Currently I do aroebic exercises (stairmaster and eliptical) Mondays and Wednesdays for 30-45 minutes before work, and on Fridays, I do a full workout for an hour with friends (who by chance will be accompanying me on the hike). My goal is a bit different than theirs. I want to shed a bit around the waist (15 lbs would be fine), but I am not interested in overall strength training, I am looking at improving endurance and cardiovascular preformance, and also doing some low impact conditioning on my legs and knees.
After only a few weeks, I have dropped 7 lbs and my heart rate on the stairmaster has dropped by 10 points/minute. I am making progress, and am enjoying being at the gym. About 6 weeks before the trek, I will add walks on my off training days. I hate to train on pavement, as it is hard on the knees, so I am trying to build up in the gym first.