Every stage of life is a one-time shot. You only get your high school years once, college, early career, kids; they come and go never to be repeated. The friends you make during each stage are special and unique, the closest ones spanning multiple or all stages.
As we get older and society changes, it's harder to make friends. Sometimes our oldest and longest friendships are still the closest, even if they are thousands of miles away. Taking a vacation or weekend to visit those friends may require hours of driving, a plane ticket, or something else.
But those friendships should be cherished, held on to and invested in. They may not be easy to maintain and you may not see each other often, but make the effort. Travel, call, text; something to keep in touch. A close friendship is priceless, be sure to treat it that way.