Dogs Merlin's age need little and often. She definitely shouldn't be going on 4 mile walks with him, as his bones are so delicate, and Goldies can suffer hip and joint problems. BUT, he needs more than one short walk a day and most importantly, a wide range of places. I don't mean travelling for walks, but you need to do town (sorry, village) walking on a lead, open space walking, woodland walking. Get them used to all the sights, sounds and smells you can. Exhaust them mentally, not physically.
We know Charl has no problems with leaving her house during the pandemic, so could easily go to safe open places with him. (Safely, and not breaking rules, please)
No, it's really not hard, for all the reasons you state above.