Which requires more effort: A 57-mile ride at 19.5 mph or a 27-mile ride at 16 mph?
The answer: It depends on whether you're riding alone or a group, along with factors such as terrain and wind.
Yesterday, Dr. Jim and I rode in a big group ride north of Indy. I didn't stay with the big group for the entire ride; I turned around (with the wind) early with four other guys. We ended up with 57 miles and an average speed of 19.4 mph in right at 3 hours of riding. My Training Stress Score (TSS - a measure of effort based on the product of ride duration x average power x a power-dependent intensity-weighting factor) for the ride was 110.
Today, Dr. Jim and I stayed closer to home and rode the Orchard route as a "recovery" ride. The word "recovery is in scare quotes because everyone knows that the terrain around B-town does not really provide for a truly easy ride. Even with the approximately 2000 feet of climbing on today's ride, we tried to take it relatively slow and easy. We averaged just 16 mph for the 27.5-mile ride, which lasted just under 90 minutes. My TSS for today's ride was 113. That's more total work in half the time.
Why did I have a higher TSS on today's shorter, slower ride than on yesterday's longer, faster ride? Yesterday, I was sitting in the back of the group, protected from the wind and only expending significant effort after turns (when the elastic was stretched) and to avoid getting gapped when we rode against a tough crosswind. Today, with just Dr. Jim to keep me company, I had to work a lot harder, even though we were intentionally taking things easy.
The lesson here is pretty obvious: sitting at the back of large groups is a relatively inefficient way of improving your cycling fitness. If you want to get better faster (or even just maintain a certain level of fitness), you've got to stick your own nose into the wind. Your ride lengths and speeds may not sound as impressive, at first, but you'll get stronger faster.
That said, it's fun to occasionally jump into a big group ride. Just treat it as an "active recovery" day than a real workout (unless you spend a fair amount of time at the front, pulling into the wind).