unless you have an endless amount of time to devote to exercise, you pretty much have to pick one over the other
That quote sums it up. I am certainly not the authority, but I think I do relatively well with both weight training and hiking. But its a major commitment. For example, I have hiked 1300 miles this year, but I can honestly say other than backpacking trips I have still gone to the gym at least five days every week consistently. In fact, I still enjoy working out nearly as much as hiking. But I don't really hike to workout if that makes sense. I like the workout, but that is rarely the objective. I often get harassed for going to the gym Sunday after a backpacking trip, or day hike, or for choosing the gym over Boulders, but I think those are the habits one needs to keep if looking for substantial gains in areas related to weight lifting. I can devote an endless amount of time to exercise because I coach. Some weekdays of mine go like this: gym with team in morning before school, practice for two hours and then LA Fitness, with a lot of eating in between ;)
Spiderlegs offers some great tips and points in my opinion. But, don't worry about a little extra mass impacting knees. If the extra mass is good lean muscle and you are training properly, your legs will also grow and strengthen along with upper body. There is a big differences between putting on five pounds of muscle and putting on five lbs of fat. I lift for strength and size all the time and do just fine, I think the hiking just keeps me lean which is nice, but I still feel strong, usually rep around 225 for bench and might on occasion go up to 275 or so for squats. But I will admit I do legs every other week now, because they are hard to plan around hikes and they immobilize me for a day or two. lol Also don't forget the body is built in the kitchen not the gym, that could explain the additional 8 pounds gained. You can keep working out at home with the pull-up bars, maybe do the kettlball suggestion etc and prob notice some nice gains in strength for sure. But if you feel you need more substantial gains or more strength, you might think about a more traditional gym and more time in the gym. The hiking will ultimately just complement all that hard work you are putting in elsewhere and I think you will be happy with the results.
Don't stress about if a 20 mile hike is better for your gym routine or physique than a ten mile hike, just lift hard and eat well the next day