Was this a fueling issue?

[rubysoho]

I don't know what sustained energy is. But two water bottles over 4 hours is not enough.

Temps were in the mid-60s yesterday, but it felt noticeably cooler as the sun started to get lower. Did you add any layers during the last 60-90 minutes of your ride?

No, and I definitely felt like I needed another layer on my core (and my legs even though I had on long/winter riding tights).