Runners are regularly subject to a slew of insults, "solipsistic" not least among. It, like many terms of disparagement, is the result of fear. A runner's self-contained disposition looks like self-worship to someone who lacks it. The presumed vanity that explains the regular pursuit of solitude. There's something suspicious about a person who doesn't need to be constantly distracted from their own interiority. What is wrong with them? If they aren't stupid, they must be solipsists.
I see what you mean. We are unusually attuned to our private well-being. We run as much as we do because it is our most reliable source of meaning and joy, the most honest expression of who we are. But on further reflection, I would hardly call this kind of self-awareness solipsistic. On the contrary, we have the perspective to observe that running is our preferred way of being in the world.
Therefore, solipsism is a charge against which I will eagerly defend most runners most of the time. But, my god, you people, do me a favor and take the damn earbuds out of your head holes. When you can't hear the person coming from behind to pass you on the single-track trail, never mind the music of the wind through the tall grass, you aren't running outside; you're attempting to make the outside the inside, which is the definitional ambition of the solipsist.
If you want to live and run in a private bubble carefully curated by you and your favorite algorithms, the apparatus of the entire world is posed to assist you. But if you want to live in that private bubble, have the common courtesy to keep it private. I.e., don't let your closed-off reality crash into our shared one.
Or, better yet, don't isolate yourself in a private space of your own design in the first place. Don't forget that the appeal of running is that it opposes that very impulse. It puts you in the world, where, like your ancestors and your fellow homo sapiens, you belong.
Turn down the interference and you will hear what the subtler parts of yourself are already telling you: The world is enough. Be in it. Live in it.