I don't know how you feel about backpacks, but that would be a good tool to enhance the boys' walk. That way you can walk a little slower for the older one and the boys will get a good work out also. Make sure that you don't put too much weight in their backpacks and you have a backpack that is right for your dog.

Another plus to walking with a backpack is that it gives the dogs a job to do. They get so proud when they can help you out.

Hope this helps, good luck!