I'll start by quoting something I posted in a thread on a similar topic earlier this year: I workout to achieve a shape to my body that reflects who I am at my core. It's an ethos, and an aesthetic. My health is part of both insofar as it allows me to enjoy the benefits of being able to live happily and healthfully within the shape that I define, and to express myself in both athletic and carnal pursuits. So my motivation is simple - do it or die a slow death emotionally and psychologically and spiritually. It's that essential to who I am. Not working out is painful for me. Working out is truly an elemental function that when missing makes me depressed and out of balance. You might as well ask me to stop thinking, feeling or breathing. In fact, I did just that for almost nine years, and it nearly killed me. Thank God I came up for air when I did.
I'd add that you need to find the physical expression that matches you at this stage in your life and helps you reflect who you are at 50, or who you hope/strive/aim to be. For me it's lifting and physique training, simply because I've always felt a little like Henry Rollins on the subject of lifting:
The Iron is the best antidepressant I have ever found. There is no better way to fight weakness than with strength. Once the mind and body have been awakened to their true potential, it’s impossible to turn back.
The Iron never lies to you. You can walk outside and listen to all kinds of talk, get told that you’re a god or a total bastard. The Iron will always kick you the real deal. The Iron is the great reference point, the all-knowing perspective giver. Always there like a beacon in the pitch black.
I have found the Iron to be my greatest friend. It never freaks out on me, never runs. Friends may come and go. But two hundred pounds is always two hundred pounds.
But that isn't everybody. For some men it's running, for some it's skating or skate boarding or surfing, some it's tennis, some it's football or rugby or volleyball or baseball/softball. The list goes on, but the body you build and shape and maintain and challenge is the body that quite literally - to my way of thinking - gives shape to your spirit.
In another post (I write on this topic with pathetically alarming regularity, I just realized) that was about body types, I noted that I'm not usually attracted to super lean guys, but one day I saw a buddy at the gym - who was once an Olympic contender for javelin - demonstrate his sport and it was as if the most beautiful man in the world suddenly emerged from this guy, and it was pure art. Not because of how he looked - although I suppose it didn't fucking hurt - but because suddenly who he was inside connected with who he was physically and he made sense as a whole person. It was amazing.
THAT is the reason to "workout", to literally and figuratively "work" to let your spirit "out" of the shell of bad health in which we can too easily imprison it before its time if we're negligent and careless. This is about the man we ache to be in spite of time and biology. Mind you, time will win, but it will let us down more gently, and see us to the finish line with grace if we accept its challenge and keep pace with it, and don't surrender simply because it feels easier now that the going has gotten a little tougher.
And while there is no winning, per se, there is honor. So I refuse to give up when the journey as I age throws more exciting challenges than it did when I was younger. Time is actually my friend. It is teaching me, and pushing me to keep pace in the way a good friend will do in the most difficult times - it shouts a little louder not to defeat me, but to raise in me the now at long last fully matured man that the boy in me could never have been. In truth, I could NEVER have trained as hard as I do now when I was younger, I didn't have the life-skills and the tenacity to push as hard. It's ironic that it happened 25 years later for me than some, but WTF, it happened is all that matters.
So find yourself, honor who you are and what you have yet to fully realize about the man you've grown to be. If not, in a way, you dishonor your youth, and disrespect the time invested to get this far. There's still time to answer the question of what you want to be when you grow up. There truly is.