When not pregnant, I've run a marathon, regularly walk 15 miles a day with a double buggy up steep hills for pleasure, do plenty of more formal hill walking. My in-laws live in Cumbria and we often go visit and hike for miles all day up Scafell Pike, over Catbells etc etc. Love it!
During pregnancy:
At 15 weeks I can barely do 1-2 miles at once. An outing to Keswick involves plotting where the next coffee shop is so I can sit down. Steam railways become appealing because I can sit down.
At 20 weeks walking for pleasure is out.
By the time I hit viability at 24 weeks I'm using a crutch, and mobility scooters in malls.
I'm nearly 32 weeks. It is agony just moving from room to room. It even hurts to sit down now.
My cardio isn't too bad though - I did 50 lengths of the pool last week. It's just my pelvis and lower back is completely screwed, so everything except swimming is completely out. You have to love PGP/SPD.
This is DC4 and it happens with every pregnancy.
Some women can run marathons at this stage. Some are like me or worse. Even a previous pregnancy when you've been fine doesn't mean you'll be fine this time, more is the pity.
Only book it if you're sure you can cancel with an affordable-to-you penalty!