Using the right equipment is important. A walking harness is your best choice for a brisk walk with a leash that is about 4-6 feet long. Walking for pet "fitness" is different than a casual stroll full of "sniffs" and pauses. Most people walk their pets covering a mile in about 25 minutes but that isn't designed for "conditioning" and keeping your pet healthy. Try to start your pet (and you) with a brisk walk requiring a little more effort. Your dog can handle this unless they are older, injured or have medical issues.
If so, begin with a short 5-7 minute warm-up. Over time, try increasing your pace to about a 15 minute mile. This pace is good for you and your pet, and you should break a small sweat doing it. On the way back, make it leisurely, allowing your snazzy pet to sniff away, stopping and casually walking back home. Pets love the athletic part of the walk as much as the stop and sniff, explore and go return. And the best part is returning home with a healthier, happier and relaxed pet!