WTWTA you said that before cars became so commonplace it would have been normal for pregnant (the OP speaks of heavily pregnant) women to walk 12 miles. My mum was pregnant in the 70s without access to a car and she certainly wasn't "normally" doing 12 mile walks or anything like that. Her work was not 6 miles away, or 12, and nor were the shops. It was not "normal" for her to need to walk this distance for anything.
My work was 15 miles away with my first pregnancy. At a normal walking speed this would have taken me 5 hours. I would not think it usual or common for a person to walk 4 or 5 hours to work, I would've needed to leave at 4am and wouldn't have got home until 9pm. This is not "normal". Before public transport people used to live near to where they worked. This type of walking commute has never been the case for all but a very few people.
And doing it when heavily pregnant as a matter of course? No! Why would they be doing it? When my mum was pregnant women gave up work quite early, before those days certainly women worked very hard on farms and in homes and factories but none of these things involved a monster walk there are back when heavily pregnant.
I don't understand this time when you imagine all these heavily pregnant women were walking these distances as a normal part of life. Or why it would be desirable that they should have to do so. In parts of the world where they do have to do so, it is not seen as desirable, surely.
You talk about walking for pleasure. That is an entirely different thing to saying that is was normal for heavily pregnant women to walk 12 miles as a normal part of their life - I just don't think that's true.