Tim,
thank you! The 50M was 4 weeks later, and I did feel more beat up at 30 miles into it than at the end of the 50k.
I would have thought that heat stress would have manifested as increased HR, rather than increased PE at a lower HR, but I see your point. My HRV and RHR had not fully recovered before the longer run.Since then, my RHR (normally 48-50) has been 55-65.
I also have Parkinson’s, which may throw things off.