Of course, just to say go ride a bike is rather simplistic. I wrote a blog post years ago for a health wellness and fitness website but I can't pull it up on my phone. I'll find it in the next couple of days and post it here. I hope it's as good as I remember it!
Two Wheels
Stay Safe
Robert
"SPINMAN"
Let me inject some reality into this discussion (from someone who worked in the health profession).
Most physical exertion is beneficial to the body while taking into consideration what it is you are doing, the duration, and/or intensity of the activity.
Different exercises affect different muscles/muscle groups, joints, etc. and so can limit one's ability to perform them depending on physical condition and injuries.
Specific activities will put more stress on certain parts of the body so you need to do different exercises to obtain overall strength and stamina. Swimming has the least orthopedic impact, and is thus a great way to maintain muscle strength while recovering from injuries aside from doing it for normal exercise/recreation; but like all exercise it does not cover all muscles nor necessarily work those muscles being used through their full range. Easy test of that statement can be done when you climbing stairs. Most people can easily climb stairs one at a time; some 2 at a time, and a choice few 3 at a time (shorter legs even harder). It has very little to do with how often you do it or at what duration if you only climb one step at a time, and then go to 2 steps since the range of your muscle strength has been limited by the previous exercise. Muscles need to be worked through their entire range of function to maintain or increase strength.
Biking is limited in what muscles it works and due to being constrained to a piece of equipment you are limited as to the range of muscle movement and thus are going to be limited to building strength and stamina due to the limited range of muscle movement. Cycling has low joint impact in some areas and more in others compared to othrr exercise so I will not say that you will not have joint discomfort just because you are on a bike. Hips, knees, ankles, shoulders and wrists are all impacted in some way, and depending on your body and riding methods might be a bad choice for exercise. Talk to your Dr. and be specific and honest about what you are trying to do and issues encountered when active in the discipline.
Good doctors/physical therapists are going to recommend a wide variety of low effort/impact exercise first bulding up to those areas needing more help. I have never had any Dr. or therapist tell me to just do one activity or bias one over another except for recovery purposes where an area that treatment is being focused upon. I have had a Dr. who, knowing my penchant for "self-destruction", instruct me to settle down and do a little less cycling and increase my exercise variety.
Note: only 150 minutes of "exercise" a week is supposed to help prevent heart disease so it there is plenty of to get your cycling in and do other physical activities be thwy outdoors or not (I go leg lifts in bed for core strength before sleep; how easy is that?).
Presently; after at least (not all x-rayed) 8 broken bones (heel to head), ruptured discs, fused vertebrae, torn rotator cuffs, bone spurs, arthritis, and surgeries; I still do nearly everthing I did 50 years ago; as I have striven to do that which I did when younger through to the present. I climbed a tree in my yard for no other purpose than to climb it for exercise. Probably not too many +60 yr olds doing that, but had I not kept climbing through the years, I would not have been able to cycle for hours and then stop and do rock climbing for hours on my recent tour last year. Sitting is early death!
Vary your exercise activities (try something new, be it cycling or something else), their duration (mix up short and long sessions), and intensity (do not "kill" yourself everytime; do some light duty sessions; or mix during same session).
Enjoy your summer/winter and stay hydrated, cool, or warm as appropriate.; but mix it up a bit and never try to over do it as that often leads to injury. Pain is a sign to stop, do not ignore it!