+1 Mr. Green
However, all that vain cover model stuff sells. It sells supplements, gym memberships, apparel, subscriptions, followers, books, etc. However, unless you have the freedom* to pursue it full time, it isn't attainable for the average Joe, especially as we graduate out of our invincible 20's. That being said, you probably posted this because you don't like the way you look in the mirror, so to a degree it is valid. There's a sense of pride and satisfaction you get when you like the way your body looks. More than 1/3 of US adults are obese, which means every day millions of people probably look in the mirror and hate the way they look. For you, maybe there's a sense of disgust and disappointment (shame) now that you see some fluff around the midsection. It's vain as hell and super subjective, but it is what it is. Will it kill you? Not yet. Does MMM approve? I sure as hell doubt it!
My high school tennis coach ran cross country from the time he was in high school, through college, and then continued running for fun with his students when he coached track, cross country, and tennis. Now in his 60's, his body is wrecked. His feet and knees have arthritis. He can't run or move like he used to. Up until 55 he was the epitome of fitness, outrunning high school kids mile after mile. Did he lift, brah? Nope. Push ups, pull ups, crunches, leg lunges, and running. In my opinion he simply went too hard, too long, too often. Since you mentioned marathons and triathlons, I figured I'd share this with you as if may affect your quality of life in your 50s, 60s, and beyond.**
So, your easiest fix is simply adjusting your diet to combat the slower metabolism. It requires no additional physical effort (lifting), just some self control and better decision making when it comes to what kinds of food you put in your body. If you can make time to lift weights or exercise three times a week for 30-60 minutes, you'll probably like your reflection in about 90 days.
When it comes to training for function or aesthetics, that's all up to you, my man. It depends on your motivation and what drives you as an individual. For me, the gym is a way to give me an edge over (or keep up with) my opponents in tennis. It is also an excuse to stay off the tennis court 'because I need to do cross training' and essentially save my hips/back/knees from getting too beat up too often. It creates balance and moderation, which I'm not wording very well at the moment. If I happen to look good, it is an added bonus, but not necessarily my objective.
*Maybe rush like a madman to reach FI, then really devote yourself to whatever your passions are. That's kind of my plan, while doing the best to maintain my fitness along the way.
**We've got better running shoes now and better technology/knowledge, meaning if you did the same thing it probably wouldn't ruin your feet and knees quite as fast.