I have just recently completed my first mile in a pool. In working up to doing the distance in a 25m pool, I counted two lengths as one, and broke it down into manageable portions. I think I started at five laps (of two lengths each) and gradually built it up. If you are like me, you will end up wishing your day away so you can get to the pool for the next session, so you can increase your 'count'.
I must add, that if you are disciplined, you can actually work through problems with your stroke as you do the 'work'. I was always weaker breathing on my right side...so as I was doing the lengths I would focus on this...I would also try and get an end lane, so I could focus my head movement as I took the breath. i.e looking down at the tiles, then turning through ninety degrees with body rotation to take a peek at the side of the pool and take a breath, then straight down again to look at the bottom of the pool. So one length would be on my easy left side, then the next on my troublesome right. Coming back breathing on my left side allowed me to reflect on what I had done wrong and adjust accordingly on the next length. If I got tired, I would stop for a few yoga breaths, and found the the more lengths I done, they eventually became unnecessary.
I am not advocating distance swimming over drills to refine your technique, but this post is addressed to those who are 'drilled out' and practice alone in a pool,and need to take the next step. Hope this is of some help to somebody taking up the challenge of a mile swim.