People immediately and correctly responded about skin damage. Many years ago, I remember a National Geographic article with photos of people from two different cultural groups (sorry, I forgot which!) and the ones who were almost never in the sun looked ridiculously younger than they were, the ones in the sun all the time looked much older. If you look at some of the formerly top long distance triathletes, their awful leathery skin is a striking contrast to their otherwise still very fit looking bodies. So, wear a hat and, depending where you are, sunscreen, long-sleeve shirt, etc... to avoid sun damage.
There are a few other considerations I'll mention, but these may be applicable so many years in the future that I wonder if you'll remember them by then(!). It gets more difficult to build muscle mass and not to lose it as you get older - read about sarcopenia - so, it is good to build some reserve muscle mass while you can (perhaps by your 40s if not earlier) and work via resistance training at not losing it. This presents a decision: you may run a bit faster with less muscle mass (Ed Whitlock) but prioritize not only looking better but functioning better in daily life, which argues for some more muscle mass. Another thing is that how you are perceived is not only how you look in a photo but how you move, etc... My friend's dad is about to turn 98 and, after walking 8 miles a day for *many* years, still moves like a young man; it is worth working on balance, for example, which tends to decline as you age. Finally, one quick off-topic suggestion: protect your hearing when you are young - and you will be grateful later in life that you did!
One other thought that may surprise you: don't assume that people at your high school reunion will appreciate your looking 15-20 years younger than they do!