If you are starting your training from a long period of inactivity, it pays to get a quick check out.
Been a couch potato? Did your idea of working out involve getting another beer? If you park two blocks from work, does that seem like a long haul? If you have been inactive for a while, one great and preventative step is to get checked out. Find out if what you want to do might kill you.
If possible, find a doctor who deals with athletes or understands what it is you are about to do. Some people may not understand why you would subject yourself to backpacking or trail running at all. Listen to what they say and start working on the basics before you start to workout.
It is possible that there are a few steps in your training before you start the actual training you had in mind.
If you have issues, or you really have no real clue on where to start training, a personal trainer is a useful person to know. A trainer can set you up with a program in steps to get the fitness you want at a rate that will make sense and not leave you with a new injury. Again find someone you like to work with.
Once you are on a schedule, you can tone down the meetings and work on your own. If you can not meet your goals alone. Or get a partner. It is harder to let someone else down than to let yourself off the hook.
Beginning to train is hard enough if you are used to it. When you have been inactive, it is much harder. So, good for you for having big goals! Get checked out and accept the help you need to meet your dreams.