I hadn't bought a heart rate monitor when I posted this thread, but I suppose the anxiety could have elevated it to pretty high when walking. I think it's mostly the fear of doing something that will set me back that triggers the anxiety.

I'll do the Buffalo protocol test tonight, and if all goes well I should have a sturdy indicator of what my body can actually handle (unless the anxiety caps my performance and introduces symptoms earlier, of course). Then I should be able to feel more comfortable in exercising below the threshold I come up with.

Did you do the Buffalo protocol thing?