The speed of your learning curve really depends on you, and what/who you have available. If you have someone experienced that can show you the ropes, it will probably only be a few overnights. If you are doing it all alone, or with another newbie, then it might take several more trips, and lots of discussion on online boards. Your first step is to go on an overnight trip. Mileage is not important. In fact, you can car camp, with backpacking equipment. You can even camp in your backyard. Gershon had excellent advice for walking a lot. Once you start getting gear, take it on your walks. When I brake in new scouts to backpacking, I take them on an overnighter that is only a few miles in and out. Then I take them on a 2 night trip with 15 miles total. Then we do a week of 30+ miles. Of course, they have someone experienced (me ) that can help and guide them. If I were you, I wouldn't hit a long trail until I had done a couple weeklong trips. They will help you see how far you can walk, plus help with what food you need. Of course, they will also help with gear selection.