I grew up swimming with my town club. I mostly raced 50m and 100m, occasionally 200m. Once each year, my club held a one-mile race at our local lido. My memory of those swims is that they were long, cold and miserable. I only did them a couple of times but it cemented in my head the belief that a mile was a too far to race, regardless of the fact that I regularly swam further than this in training. The idea of swimming 5km, 10km or longer in one go was beyond comprehension.
Fast forward to the present day and, truth be known, I still find distances of more than 400m intimidating when swum in a pool. However, I now think of a mile in open water as a short swim.