YOU need to be on an exercise routine (picture a large finger pointing at you)! You probably already are, but it sounds like cross-training could help. I'm lucky enough to have a place where I can do 6 to 8 miles 6 days a week carrying 18-20lbs on trail close by (for me that's 1.5 to 2 hr per day cardio). I'm also self employed and set my own hours (and they are weird hours). I would find a way to get some hiking in on a regular basis (between mtn. biking) and as OM said make sure you are stretching before and after everything. If you can't find hills to go up, maybe stairs or bleachers...might even be able to do these at night if lit up. Having trouble getting motivated? Find a partner with similar goals and get out there at night. There is nothing like hiking or running to get into shape for backpacking. I don't ever remember feeling leg muscle pain and I'm now 56, but I also drink plenty of water and stretch out during longer/harder backpacks.