The crux of the problem is that humans are notoriously bad when it comes to perspective, and as a result we can be quite impulsive. As a Princeton psychology study explains,
Impulsive choices or preferences for short-term rewards result from the emotion-related parts of the brain winning out over the abstract-reasoning parts. "There are two different brain systems, and one of them kicks in as you get really proximate to the reward."
We know that being active has all sorts of awesome benefits that we want, whether it be longevity, health, strength, fitness, or better appearance; the problem is winning out over our emotional predisposition to short-term pleasures like TV, games, etc.. There are several popular ways you can approach this problem and "trick" your brain into choosing exercise.
Make Exercise a Game. Since we are predisposed to doing immediate pleasures, find forms of activity that you actually enjoy doing (in your case trail hiking) and do them a lot. This is most commonly sports like basketball or soccer, but can vary wildly depending on the individual.
An alternate but similar approach to the gaming aspect is using something like the fitness site Fitocracy. They make fitness into a social game, where you compete against your friends, receive immediate rewards (in the form of achievements/badges), and being held accountable to your actions by showing them to the public (see next point).
Hold Yourself Accountable. If you hold yourself accountable and treat exercise as an actual requirement (as opposed to something that is "nice to do"), you are far more likely to follow through with your commitment. Put your workouts on your calendar, punish yourself when you miss workouts (no tv for a day), and make your goals known to the public. If you tell your friends that you are working out, ditching on your goals now has the added burden of looking like a quitter to your friends. Get a workout buddy who holds you accountable, the kind who would drag you out of bed in the morning for a run.
Reduce the "Short-Term Cost". The lower the burden exercise places on you, the less likely you are to put it off. Start small, build big. Do a handful of pushups here or there throughout the day, and make that your thing. As time goes on, add more reps / sets. As more time goes on, add new exercises to the routine. Before you know it, you are doing actual workouts.
Realize Short-Term Goals. Since we are so bad at seeing long term goals, you need to start looking at exercise from the short-term perspective. Set daily goals (do one more pushup than yesterday). Look at yourself in the mirror before/after working out. Watch motivational videos to reinforce your long-term goals and keep you motivated. Basically, if you find yourself about to skip a workout, do everything you can to convince yourself to stick with it.