You're making an erroneous assumption. When these riders go out for 2-3 hours at 19-21mph, that is their zone 1 ride. They're not in zone 3 and they're not working as hard as you think. That, and are you with them 24/7? Perhaps they only do steady state riding when they're on rides with others (you) and the more intense rides when alone or with teammates. Most high-end racers ride alot alone or only with comparable athletes so they don't risk being crashed out by someone not at their level and lose a couple months of their season. Here in SoCal, where there's a cat 1 or 2 around every other corner, I rarely see them out riding even though I'm on the road 6 days a week.

As for you, and what others have alluded to, you need endurance. Do the intervals on top of that to improve speed and ability to respond to accelerations. Do the endurance rides one day, intervals another. If not already, I'd recommend you make sure to ride 5-6 days a week with hi-intensity intervals only two of those days.