Going back to the OP's question, I think planning is essential to get full value and experience from your ALR,
But you can over-do it.
The first time I did an 7 day ALR, despite having planned every journey on every day for weeks ahead, I spent most of it pouring over timetables, maps and live departure boards on my lap-top and phone that I forgot to enjoy the journeys I was making at the time.
It became like work! After the week was up, I realised I had been everywhere and seen nowt.
It was only on my second trip, again planned 100%, that I relaxed and was flexible within my plan and so really enjoyed myself. If I didn't get a planned, or even booked train, it didn't matter, there was another one along soon.
(Oh, and try to persuade the folks to allow you a couple of goes on the sleepers.
With 1st class, you will have a berth to yourself and a start point for the next day anywhere from Fort William to Penzance - this opens up many more routes to you.)