Walking on treadmill vs outside

I have purchased a treadmill for my home because I am unable to walk outside on walking paths or just sidewalks for more that two - three blocks, even with portable oxygen. I have to stop every two blocks to catch my breath.

While walking on my treadmill, I walk at least 45 minutes a day at a pretty good clip and in the last few weeks I have added incline increments. It is not difficult and I really enjoy it.

My question is: Why can’t I walk outside on all flat terrain with portable oxygen for 45 minutes like I can inside on my treadmill?

I live in a beautiful part of the country and the summers and falls are perfect for outside walking. I am missing out on some beautiful walks around the lake with family and friends because I just can’t do it?

Thanks,

JWG