People, especially older people discount exercise intensity. They think ANY movement is better than none. Not true. To be beneficial, exercise must take you to at least 65% of your maximum heart rate and stay there for at least ... ~20min per session x 3 sessions per week. That can be worked up to, of course. In my opinion, it is hard to give the necessary attention to achieving decent exercise intensity when one is navigating the peculiarities of a bicycle AND dogs, joggers, children ... the best way for older adults to get cardio exercise is indoors. At a gym or at home but inside, on a stable piece of equipment like a Rowing Machine, Elliptical Machine, Treadmill or Upright or Recumbent Exercise Bicycle. Many now come with LCD panels and TV or PC interfaces if boredom is an issue. Trust me, this is better than flailing around outside at a walking pace on a bicycle. A one or two mile walk in the park together is altogether better than a bicycle ride. You can get done in two miles walking what will take 5 miles or more of bicycling. No risk of falling, no gears or other stuff to manage. FWIW.