Obviously getting fitter is goal #1 but don’t forget about technique, tactics, and strategy.
For example, look at the 30 minutes before the first effort. Are there ways to ride that more efficiently at a lower power output but still be with the group? For example, better positioning in the group, getting on good wheels with better drafts and who ride at a steadier pace? Are you being as efficient as possible with your drafting technique? (e.g. positioning, judging cross winds, etc). Are there little climbs in the miles before the big ones where instead of powering up at the group pace you could move to the front beforehand then drift back through the pack? Are there spots where 30 seconds of serious work will make it easier to stay with the pack for the next couple minutes?
A huge trick to bike riding in groups (race or ride) is not just getting stronger, but learning how to use the least energy possible. Cycling is actually a lazy man’s sport, not a tough guy sport 
Being lazy is a skill not a character defect.