I've been riding bicycles since 1945, still do, but I learned a long time ago my body adapts, it becomes very efficient at repeating the same exercise. What was difficult became easier, I needed to switch cardio with a different type of movement, and intensity to improve performance.
By doing the same workout you are flirting with overuse and chronic breakdown of muscle, tendon, and bone. Too much of the same repetitive activity or stress on the body will not allow adequate recovery.
If you mix it up, your body is fighting harder to adapt, recruiting more muscles which results in better performance. Weight training has taught me to change routines, I get too comfortable doing the same lifts, I have to work harder doing new routines.
When I started riding the Me-Mover about 2 years ago, my legs were on fire, the lactic acid was letting me know I was doing something different, so different I had a hard time going around my apartment complex. I figured at 78, it's gotta be the age, so I fought the lactic acid and went beyond the plateaus. it worked, I adapted. At 80, 30 or 40 miles is no big deal, even though I'm using more energy to propel the Me-Mover than my road bicycle.
That comfort level is hard to break out of, but it's worth it.