I was fortunate to take swim lessons from a professional coach/instructor for free where I live. According to the instructor the number one problem people have who complain of being tired (baring medical issues or grossly out of shape) is body position in the water. Most people who are not swimmers swim with their heads out of the water. The instructor said, "head up, feet down". This causes the body position to be diagonal relative to the surface of the water which causes greater resistance while swimming which, in turn, makes you tire easily.
For the freestyle the water line should be at the top of your head in the center between the front and back. You should be looking straight down to the bottom of the pool. This is why swimming lanes have a "T" at the end of the lane's black tiles. Competitive swimmers know they are at the end of the lane and to start their flip turn without having to look ahead to see the wall.
When I finished the class it was amazing how much easier it was to swim laps. This was the case for the other four strokes we learned (back, breast, side, and elementary back). The breast stroke is an exception where the feet are lower due to the head being raised up to breathe. But, the slightly inefficient body position is counter-balanced by the legs/feet providing most of the propulsion (in the freestyle it is the arms that provide the power). There were also many other things I was doing wrong that I was able to correct. A good instructor can help you swim with the correct technique which will greatly improve your enjoyment of the water. I'm glad to see you are pursuing swim lessons.