[Archaea]

Just a new set of muscles. Eventually, you won't notice it.

Swimming is actually the least demanding once you acquire some fitness. However, you're using upper muscles which you don't use in running or biking. Biking is also not too demanding. Hard running wipes me out. I did just three ten minute tempos this morning and am wiped out.

The swim is fun and delightful. However, it appears you are inefficient in your stroke and therefore you are fighting yourself. I'm just stating that based on time. Open water swimming can be fun.