Really, it's a wonderful thing to tire out pups with play and walks! So, I would not change anything you are doing!
However... When we had our first Berner, I walked her twice, sometimes 3 x daily. What puppy wouldn't like that, right? Well, apparently, not Berners, with all their joint issues. Actually, she let me know she didn't like walking so much by sitting down and absolutely refusing to go any further! At the time, I referred these "troubles" to pro trainers and they thought I was nuts! (A puppy that didn't like walking???) Anyways, long story short, after 2 more Berners, (and having had 3 other mixed breed dogs in years gone by) I realize that the Bernese has some special issues that require special care. In my opinion, a Berner is A-OK as long as, just like any pup, you gradually increase their endurance for hiking, long walks, etc. IMO, free play is ALWAYS good, no matter the duration,
I'd wait until the pup is at least a year and then, don't go for TOO long a hike and watch for any limping afterwards. Of course, keeping weight down is good, but from reading your post, you are already doing that. (my rule was 7 mo = 70 pounds). Our younger dog (aged nearly 3) happily climbs up steep trails with us for 2 1/2 hr hikes, but he has been gradually accustomed to this. In the heat of this past summer (SW Canada), there were a few times when we did short hikes (45 min) in the early evening (when I felt it was cool enough for him) and he simply lay down and decided it was too much for him to continue. Made for a very long (duration) walk but I did not push him lest he suffer heat stroke. I am more comforable hiking with him in the cool - fall, winter and spring months. Then, I can take him for long hikes. (our female has joint issues and sadly cannot go for any long exercise.)
I always walk our dogs for at least an hour in the morning (it's a bit of a stroll for them, more of a sniff adventure) and then take the younger, stronger dog for hikes in the evening and on weekends.

Hi everyone,
I was just wondering how much exercise to give my now 8 month 76 lb berner. I know I don't want to overdue it so as not to hurt his joints but I'm not sure he is getting enought either. He gets free play time in my backyard plus about a 20 to 30 minute walk 4 or 5 times a week. Lately, he has also had a couple of playdates with another berner and plays so well with her. Comes back nicely worn out . Do you think it is ok to increase the walks at this point? I'm am just trying to avoid joint issues later if possible as we all are lol. Thanks, Katie Palmer