Again, I have to second what W_D has mentioned, that is, for conditioning, you really don't need to carry your fully loaded pack, and I also agree that there isn't a really good reason to do that. Sure, you want the pack loaded, fitted, and tested before you head out, but you don't need to carry it several times for several miles just to test that out.

Walking, I think, is the best conditioning you can do for backpacking. Walking up and down hills is especially good. I'm lucky in that regard because I can hike in my backyard and get some pretty good climbs in a short distance. For me, biking isn't a good option. We don't even have a shoulder on our paved roads here, and the mountain biking trails are lined with shards of sharp edged chert, so I don't bike anymore.

If you'll be hiking where there are lots of hills I'd also suggest you do some conditioning on trails if you can. Walking on trails will help get your ankles in better condition that walking on pavement.

For me, the ankles are among the first parts to complain if I'm carrying a pack when I'm not in good condition. This never happens if I do a few miles a few times on our rocky, hilly, trails here before I backpack.

And I'll point out that I don't think you need months to get in good enough condition for a 20 mile backpacking trip either. If you get out and walk a few miles a day 2-3 times a week, you'd likely be ready in 4-6 weeks, at most, to do a 4-8 mile hike with your pack, each day for 2-5 days, without busting yourself up.

The main thing is to include flexibility in your trip plan. Don't plan on doing a lot of miles, or even making it to a specific destination. Instead, plan on stopping to rest whenever when you get sore or fatigued. Plan on setting up camp at a certain time of day instead of a certain place. And plan on making the trip enjoyable, as opposed to an endurance test.

If you plan as mentioned above, you might well find you can easily do 10-15 miles in a day, and that you like it. Or, you might find you only did 2-4 miles in a day, and still liked it.

One thing is for sure, the more you walk, and the more you backpack, the further you can go. But that still doesn't mean you have to go far every time. Often times I find a spot early on and decide to stay there and explore it detail.