How do I untwist a very long, very twisted Ethernet cable?
Well, the good news is it does not appear that you've passed into &quot;throw it away&quot; levels of twisted yet. I've had users do things to innocent cables that I can't quite believe. I find out about them when the cable fails, usually.
Find a stairwell, or some other place you can let the cable dangle, hold, and wait. Since you probably don't have a 45m stairwell handy, you'll have to work in sections. Starting from the end is a problem unless you can let the whole thing dangle - you want to start as far into the middle as you can and let the end spin free, so that the twist gets &quot;worked out&quot;.
Another approach is to wind the untwisted section of cable onto a core of some sort (essentially a small cable spool) and to spin that as needed to remove twist as you wind it on - you'll need to keep light tension on it so it can't snarl - you can tape it if you want to pause while working. Important to then unroll it, NOT spiral it off the side of the spool. Still likely to be easiest if you can hang the unspooled cable as much as possible so it twists freely.
Once you get it sorted, you may also want to look up various methods of coiling cables without adding twist when coiling. Various ways work, and the one I prefer may not be the one you prefer.