To be fair, I think I've misrepresented the concept by wanting to be a hermit: A third place is somewhere you go that isn't work or home, but it's conventionally defined as a neutral social space where you can interact with strangers. If that's biking or the pool, then you've found your third space! If not, maybe it'd be fun to experiment with if you have the spare time :)
An important aspect of "third places" is that they are place where you don't necessarily interact with "strangers" but more that there's a steady set of individuals who, while they might not be friends, are at least people you know and have expectations of. The seminal book Bowling Alone is about the decline and fall of third places in American society and the effect it has had on health, culture and politics.