Sry if its unrelated, but is it a group ride. Im about ready for racing, and do about 50km solo rides atm at a reasonable pace and find my legs a bit tired. Had my first group ride last sat, did 70kms total (10 warmup) and a few solo at the end, but it felt so easy, i rode the front a bit and found my rhythm pace to high.
Moral of the story i found, group rides= easy at the back if u need a rest.
Other advice that i know is bad but worked for me(20 physically fit remember that). --> because i could not string more than 2 weeks good training together, i could only ride about 30km tops without serious fatigue. After 3 rides in the saddle after a month off, i forced myself to do 50km rides with the aid of energy gels, took me 3-4 of those and after a few weeks of on and off, i blitzted that group ride of 60km and now ride 50km solo at a decent pace. I only resorted to forcing myslef to 40% above my limit because of lack of time, not recommened, but worked for me so dont count it out. 3 weeks is plent of time, all important dont train over what u know ur body can recover at.