Yes, I'm fine at rest but any exertion tires me out quickly. I do life in 10-20 minute bursts and then I have to sit down or lie down.
I have been diagnosed with 2 issues which apparently cause this:
1--Cardiac preload insufficiency which means my vascular system doesn't not return enough blood to my heart when stressed/exercise.
2--Reduced mitochondrial effectiveness which means the mitochondria in the cells of my skeletal muscles no longer utilize oxygen effectively.