Why is it that I can ride a stationary bike and be fine but not be able to walk up a hill?

So I have a strange problem.
Whenever I go to the gym I can run do the stationary bike on the higher settings at 80-100 rpms easily for 15 minutes and sure it will makes me a little tired but the machine with it's pulse reader and such always tells me at the end of my work out that I am in the "Very Fit" range, and I agree, my legs don't get tired and I am usually not out of breath (and yes I am putting it on the higher settings as I have tried a ton of machines and i make sure the digital difficulty bar is near it's highest)

But when I have to walk up a hill or something, my legs again do not get tired but I am always always always out of breath, as if I had run up the damn hill....in fact it is usually easier on my body when I do run up the hill instead of walking up it.

WHY!?

why do I feel less tired and less out of breath after I bike at hard difficulty for extended time or run up hills than I do when I walk at a leisurely pace up a hill ?

this makes no sense to me....

I have really strong legs also. I can lift a stack set at 14 (i assume that is 140lbs) with one leg and a stack set at 22 (i assume 220lbs) with both legs quite easily. I also can do the full 200 something lbs on those ones where you squeeze your legs together and push them apart (hip abductors). I can do the whole stack and then some (i add more weights) when I do the butt leg lift machine. I can go between 12-15 on the ones when you bend your legs at the knee (pushing down and up...like a dolphin kick)...

I have strong legs....and my legs do not get tired when I am walking or working out usually (I used to swim, do water polo and do downhill ski racing so that is why I have strong legs)...

But could it still cause me to have a loss of breath just because some muscles are used and others are not used as often, cause the muscles do not get tired...just my chest?

There could be a multitude of things that cause this. First, never trust the pulse readers on a machine, as they hardly ever tend on being right. Secondly, if you bike a lot, you will tend to have very strong quadriceps muscles as they are the primary agonist during bike riding. One drawback to bicycling however is that it tends to shorten the Rectus Femoris (the quadricep involved in flexing the hip). This could cause you to feel tired during walking up hills because when walking, you are emphasizing your gastrocnemius (calf muscle) a lot more, and this is not improved by bicycling alone. Even though some work will be done by the calf, not near as much benefit is seen in the calf as the quadriceps. This might be the problem, and if you see you have a muscle imbalance between your calf muscles and your quadriceps, I would recommend trying to balance those muscles back out. You could do things from running on the treadmill on an incline, to walking up and down stairs, to doing calf raises, etc. But, I would check to see the condition of you calf muscles to start out with. Also, your body just might be accommodated to stationary things, and by actually moving you are introducing a new challenge that your body must try to adapt to. One more thing lol, is that by training at the highest intensity on your stationary bike, you are training your body to be efficient at short, high intensity bouts of activity, which could explain why sprinting is easier than walking. To fix this, I would add more aerobic training into your regimen, which could range from running/walking longer distances, and lowering the intensity on your bicycle and trying to ride for a longer amount of time. Sorry for the long read but I hope it helps!

Well, I can tell you from personal experience that, although I run 5 days a week up and down hills, when I walk the same route I am exhausted. I'm guessing that it utilizes different muscles and that your body becomes more efficient (ie., using less energy) to do tasks that you repeat over and over, so that when you do something different it taxes the body all over again.