They are known like with other large breeds to be disposed to hip and elbow displasia. Even if parents were healthy and good scores they can undo all that work by over exercising. OCD is another thing to think about.
My breed i'd not be doing any more than 30mins a day on lead, and that would be split into a couple of walks. Play time in the garden and going to classes too. There would be a bit of off lead running about too, but taking that out of the on lead time.
But Saints they are a larger, heavier breed than mine and there is no way i'd be walking miles especially on terrain like that.
It is the golden saying of 5 mins added per month, this is a guideline. I'm surprised that their breeder didn't mention this at all to them?
So all in all you are right to be concerned.. Good luck..
http://www.englishstbernardclub.co.uk/page_616609.html - Even on the club site there is a little bit about exercise.