What to expect...

The closest I've come to an open water swim was a few years back when I was an adult leader at a boy scout camp. I joined in their mile swim. Their lake was really just a small trout pond, probably 200 yards wide. The swim was five across-and-backs. It was cold (it was in South Dakota, and the pond was fed by snowmelt runoff. Temp might have been about 70 degrees.) I was the only one to finish. Recently I came across an upcoming 2.4 mile swim in northern Colorado. They say the water is usually 65-70 degrees at the time of year this is scheduled. They have both wetsuit and non-wetsuit divisions. I don't know why, but this one is calling me. I do 4200 yards per day, 6 days a week. I do that in about an hour (or more often 1:05) including rests. I have no doubt I can do that non-stop (I actually have done that on a few occasions.) So I'm not worried about the distance. But the cold! I just don't know what to expect. I don't want to use a wetsuit. (I don't want to put out the money for a one-time whim swim. And being 6'6", I wonder whether I would find a good fit anyway... I know nothing about wetsuits...) The pool I swim in usually has a water temp of 82-84 degrees. Once the temp was 79.1, and that was chilly but did not negatively impact my swim that day. (In fact, it was invigorating and I did one of my better swims that day.) I seek voices of experience here. Will water that's 15 degrees colder than I'm used to sap all my energy? Am I wrong to assume that my ability to do the distance in a lap pool means that I can do it in a cold lake? Can I reasonably extrapolate my times from the pool in any meaningful way into an open water (fresh water) prediction of time? (For instance, I could guestimate that I might pick up a second per flip turn in the pool, so absent those 168 flip turns, maybe my open water swim might be 3 minutes longer or so.) I'd love to hear from some people who've "been there, done that."