Definitely, healthy people should be able to hike 8+ hours a day for days in a row with minimal difficulties. Walking shouldn't raise your heart rate that much unless you're walking uphill, a healthy heart should be able to come back under 100bpm fairly quickly too
"healthy" is a very loose term it seems. I know many people who would be called healthy by their doctors simply because they didn't have any concerning medical history or present concerns.
I think I may agree with you that "healthy" should include basic levels of fitness and not simply !(ill)
