Suggestions to drain black lab's energy?

I have a 10 month old lab/dane mix named Duff and I also have a 14week old shepherd mix named Armando. Before we got Armando, we took Duff for looooong walks (3-4miles a night) and that worked just fine for him. But now Armando's in the family and I want him to get exercise as well but he obviously can't keep up with Duff. Should I go on 14-week-old-puppy-appropriate walks and have Duff pull my son's wagon with a harness for the extra exercise? Or should I just take seperate walks? Or should I get Duff a weighted backpack? I was really wanting to be able to just take the 1 walk so Duff and Armando can socialize with each other and my family together. Any suggestions?