Andy - I've been thinking the same thing. I went out for a lovely 10 miler yesterday. Unexpectedly wet to start, then glorious for a while and it did me so much good for my mental well-being. Social distancing was awkward at times, but understood with a few body language signals.
My parents are both recovering from cancer and live in Weymouth in their 70s. The outlaws are over 70 too in Marlow. For my parents we've already talked about them getting out early in the day for a stroll on a deserted beach. How important the period is from leaving the house and getting back in, stripping of clothes etc.
For the outlaws the chat we had today was similar about getting quickly in and out of the house and up to the Chilterns. Plenty of Social Distancing and no stopping at pubs.
There is absolutely no way of policing this. It will 100% do more good than harm in curing cabin fever. Take whatever precautions you see fit. Stop, walk slowly, move left, move right, go backwards, vaseline your nose... whatever you do smile and enjoy some vitamin D and be safe.