I have done, a fair few 7 day ALR’s. Both Standard and First class, did standard class when younger, with some nutty weeks out, one mad week of travelling, was when just for the sake of it, i attempted to see if could get near or break the record for how many miles done in a week, I got to around the 10.500 miles mark, living on trains for the whole week, I used the, ECML during the day, with 20 mins approx between trains, and the WCML overnight, in sleepers and seats, London to Edinburgh or Glasgow and v.v, was zombie pretty much by end of it, and was lucky pretty much every train was on time, have to say those were the days.
Nowadays, I go First Class when do 7 day ALR, but still stay out for the 7 days, using sleeper berths for 5-6 nights and seats for one night if needed to, the pace is more relaxed with longer journeys on each train etc, and use First class lounges to relax between trains, and at Euston, have a shower and clean up, I still do about 7000 miles plus in the week. I would go with what other people have said, and buy a Scottish rover ticket if doing Mallaig etc, as a 7 day ALR wold be a bit wasted if doing the long slow journeys in Western Scotland.