Yup, ditto, water, water, water.

After spending last weekend at 8,700 ft, I'm beginning to think that
the effects of altitude aren't linear - i.e. there's some point up to
which you can handle it (coming from sea level) but it gets harder at a
much faster rate after that. I was at 7,000 ft today and felt like I
was at sea level after the misery of ~9k.

6k-8k sounds like our neck of the woods, or maybe Utah....

bt