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Lovely river at one point surrounded by lush green grass and not a human in site
That could be the Annan River for about 15 miles (south of Beattock summit).
Or it could be the Clyde, for about 25 miles (north of Beattock). I think the last time you'd cross the Clyde before Glasgow is at Carstairs, where it's still a modest little river; on the souther outskirts of Glasgo you cross it again at Uddingston where it is a much more substantial river, but neither of these crossings are comparable to the mighty and majestic Clyde which you cross when making the final approach to Glasgow Central Station.
. . . The distance must be substantial because even in a pendo it takes over an hour
. . . .
Any info on this stretch stations landmarks places to stay etc
Yes, almost exactly a hundred miles.
There's plenty history in the region; old battle grounds, industrialisation, the towns of Lockerbie (the air tragedy), Moffat, and the progressive industrialisation of the whole Clyde valley from Lanark, through Wishaw to the rise of Glasgow as a significant global port. Plent reading materials are available and should provide plenty interest.