I tried doing that today. I had a 2-hour ride on my schedule, at "event pace". For the first hour I tried to stay at about 13 mph; during the second hour I worked at pushing for 15 mph as much as possible.
Riding 15 mph for more than a short time gets my heart rate into zone 4, or lactate threshold. At least, I think I'm at LT. I could tell I was working hard; I doubt very much I could have carried on a conversation; and yes, it was rather uncomfortable. But not distressing. My legs didn't feel "on fire", though. Just slightly burny.
The clouds were building up, and I'm glad I didn't get "burned" by bad weather.