I know what you're talking about when you say walks are getting mundane. I've written about my experiences all over this forum. We would go for 2-3 hour walks and cover multiple miles at a brisk pace (No joke! I would come home drenched in sweat from just walking). Over time, they got very boring for me, and when I got home, the dogs would act like they didn't expend any energy. Once they were old enough to run, I would run with them which helped things, but my Siberian was just too fast for me to keep up with him. That's when I discovered "dog powered sports" also known as urban mushing: bikejoring, dog scootering, carting, skijoring, and plenty more. I already had a mountain bike, so I just needed the appropriate dog gear similar to what sled dogs wear and use. It cost me maybe $150 total for everything I needed for 3 dogs. Then I learned the commands to stop, go, and turn left and right, and the leave it command (WHOA, HIKE, HAW, GEE, ON BY). It took each dog less than 5 minutes to learn these commands. Now I bikejor or dog scooter for 5+ miles (14 miles is our record to date), 6 days a week, and it takes us about 45-60 minutes (depending on # of bathroom breaks). It's an amazing change that saves me time, and wears the dogs down much faster, plus they're learning and being reinforced on obedience commands constantly during each run, so there is both physical AND mental stimulation. They get to "run free" at their own pace, and they're pulling around 367lbs such long distances to help build muscle and burn fat. It's fun for me to get that sled dog experience when there isn't any snow on the ground too!!