@fimm: my friend who lives a very near Hillend but a few hundred meters into Midlothian can't climb Caerketton (in Edinburgh)
I'm not sure that's an accurate interpretation of the regulations, or the latest guidance as posted by chdot above. AFAICS there's nothing in either which restricts where your exercise could take you, so long as you end up back where you started. And as you said yourself further up: "once you have started exercising you can go as far as you like". In that respect AIUI the regulations and guidance are unchanged from what they were under the Level 3 restrictions, under which at least one regular CCE poster resident in Balerno (Edinburgh) has reported multiple outings by bike along the A70 in to West Lothian and back - which IMO were entirely legitimate.
My understanding is that your friend near Hillend could do any of the following while remaining compliant with the regulations & the specific guidance about exercising:
1) Drive to Castlelaw (which is in Midlothian anyway) and walk in to Caerketton from there, so long as they returned to Castlelaw;
2) Drive to Old Swanston, which is in Edinburgh but less than two miles by road from the Midlothian/Edinburgh boundary, and go up and down Caerketton from there;
3) Walk from their home to Hillend, go up Caerketton and walk back out to home via a different route e.g. via Boghall Farm (though the walk back along the A702 in that case probably wouldn't be the highlight of the outing).
That said, one or more of the above might be regarded as being against the spirit of the broader i.e. non-exercise-specific guidance which states: "Although you can leave home for these [examples of reasonable excuses], you should stay as close to home as possible."
I am, though, entirely happy to be corrected if my current understanding of the rules is demonstrably flawed/out of date.