Exercising a hyper puppy

My 4 month old lab/St.Bernard mix puppy has a lot of energy! I have taken her on walks before but I'm wondering how do I know if it is too much? I want to tire her out but I don't want to do any harm to her either. I have taken her on 20 min walks and she does great, but does not really seem to tire out at all, I did take her on a 40 min walk a week ago and she didnt slow down or try to stop, she did whine a bit at the end but it was hot and I think she was thirsty. Is 45 min too long for a 4 month old? I feel like she never gets tired!
I do have other dogs she plays with constantly, but she never tires from that. She also likes to swim but that is something we really only do on the weekends, shes a great swimmer!

Any advice would be great, thanks!

 

Cycling or long walks could help

Have you tried taking her for a walk while you are cycling on your bike
(on a leash)? It might be confusing to begin with but she will soon enough learn to stay on one side of the bike and follow you!

Another thing I found helps is long walks in the countryside where she can sniff/chase/run/catch etc!This would turn out even better if you have a friend with a puppy who can join you.I did that last weekend and my hyper cocker seemed like he was in paradise! (make sure you bring along some water though as they get very thirsty!)

Good luck!