Thanks everyone - a very interesting series of replies!
I'm sorry I'm being lazy and not replying to each of them individually (tired from all the walking - LoL!)
The general consensus seems to be that it won't be doing me any harm, and might help, albeit not to get my hopes up. So it seems worth persevering.
I already have some physio-prescribed exercises, although these were devised in the context of my condition some months ago, and might not reflect subsequent deterioration.
I'm trying harder to do them this year (New Year's resolution), and one of them was walking. Not just doing it, but concentrating on putting my heel down first, to get the calf stretch, as I was tending to put toes down first, which is wrong.
Although nothing obvious has changed since last year, things just feel tireder more quickly. It seems to be that stamina is reduced, rather than any tangible difficulties with balance, range of movement etc. I can walk quite fast, but quickly feel as if I've been running, rather than walking.
I can't rule out that I'm just having a rather bad week, as the first week back to a worrying time at work after almost three weeks of being idle may not necessarily be the best time to introduce new exercise as well!
But I know that if I wait for the "best time", there never will be a best time - I'll always be moaning I'm too tired, or too busy, or too stressed, or whatever. So nothing else for it but to put on my goosedown filled coat (like an eiderdown, with sleeves), and just get on with it.
Yes, it has been horrid out there, so I've been setting a cracking pace - mainly to make sure I'm home again, before I get soaked. This race to beat the next downpour is probably contributing to why it's a challenging walk.
On the positive side, I have not yet had to give up and turn back, so I can still do it. I'm just aware of how soon I'm feeling the strain, compared to how far I used to get, before beginning to struggle.
Thanks again for all the replies - I will try to keep going with it.