A better solution would be to add a bike training routine to your dog walking, strength training, and outdoor hobby routine. The dog walking probably involves less aerobic peak exertion than a bike ride potentially can do, although the dog has the benefit of being more likely to drag you outdoors when the weather isn't perfect or you have distractions. That said, we all have a finite amount of time for outdoor exercise, so you're optimizing the best you can. If the dog is keeping you active in a way the bike didn't, you're making a fine choice. Just maybe incorporate some uphill sprinting or agility exercises in the walks if the dog agrees.
Keep in mind that about the time the dog reaches 7-9 years old it might no longer want to do 4 miles per day, and this will slow you down as well. At that future time, you can withdraw your deposit from the Bank of Craigslist and get back into cycling on a very similar fancy bike.