Most people walk their dogs for the purpose of elimination; these walks often take no more than 5-10 minutes, if you are walking you dog for exercise, than of course it would depend on the size of the dog. A small yorkie doesn't need as much exercise as a lab for example. I think yorkies "need" 15 minutes a day, just to explore the world, and anything over that is good exercise. There are other forms of exercise you can do, such as fetch. I think walking is one of the best forms of exercise for a human, but I doubt if many people here walk over half hour a day, if that. At 110 degrees here, Joey is only getting 10-15 minutes.
The walking should be something you enjoy, don't let others make it into a chore.
Probably the best form of weight control is through diet with exercise, but it sounds like they are getting more than enough exercise. When walking Joey, sometimes he would just stop and sit, so I carried him for a few minutes, until he "recovered" and then we continued our walk. Joey was overweight for a while, (after a tricks training class) and the vet said to supplement his dinner with green beans, they make him "feel" full. The "petite" frozen beans are very nice and don't contain the salt that canned beans do. I cut his breakfast in half, I was simply feeding too much, and also his dinner, but with the dinner I added 7-9 green beans that had been microwaved for one minute. His weight has stabilized, and we have continued the diet because it keeps him at a healthy weight.