Distance swimming almost automatically is a zone 2 activity. Each of us has a 'natural' speed through the water, mostly dependent on our length (in the sense of waterline, fingertips to toe tips) in the water (same deal for ships: google Froude's Law) but also how streamlined we can make ourselves (pay attention to what the water is doing around you - easier without a cap and as minimal a suit as you're comfortable in) and an efficient stroke (google new paradigm swimming).
I say 'automatically' because if you go faster, you won't make your distance, and (speaking from experience) you do not want to be in oxygen deficit if you accidentally try water breathing. Fine in the pool where there's an edge nearby, open water not so much. Not drowning has always been one of my key goals - and I'm a 'sinker'.