On the other side, I'm as unfit as it comes, no excersize ever in my entire life pretty much, was bed bound from almost beginning to end of pregnancy in excruciating back pain, gained 4 stone, zero of it muscles, knees completely fucked to the point I can't even squat without pain let alone get back up, I was 27 when I gave birth - started getting contractions 6am, didn't dialate at all until 3pm, dilated from 5 to 8cm from 6pm until 8pm, and gave birth without any intervention, only gas and air, after pushing officially for 5 minutes. I did basically 2 pushes and baby came out. Bang on average birth weight, healthy happy baby with no complications at all.
Not to say you're wrong, but if you were right I should've had a terrible birth. I just pushed as hard as I possibly could, all doctors around me telling me it was too early, yet my body was saying otherwise - couple big pushes and I felt baby come out.
I didn't feel put out at all energy wise and found the pushing and contractions easy. They fucking hurt, yes - but I could've kept on for another hour if I had to. I mean I usually get out of breath just walking up the stairs. I couldn't run for a minute - but giving birth felt like it had no correlation to my fitness whatsoever, I was barely out of breath. It isn't quite an endurance sport in the same way as actual endurance sports.
On the other hand you have very fit and active people, smooth pregnancy and terrible scary birth. I don't think sedantary lifestyle has anything to do with it to be fair, humans have always been the animal that struggles most during birth because of our birth canal shape and the size of our heads. Birth is no harder now than it was 100 years ago when people were generally more active - in fact the doctors usually op for c sections when the baby is larger just incase there are complications, to cut out the dangerous middle man. Women have a much better chance at survival during birth these days and I'd think evidence points more towards intervention being a large part of that.