You are recovering from cancer and you want to go faster, man - be happy your beating the big "C".
However if that is not enough, on a group ride stay in the draft, not at the end of the slinky but in the middle, save your energy for the hills so you can stay with the pack and not have to burn matches to catch back up. Stay out of the lead, learn the art of drifting back in the pack when it is getting close for you to lead. Push yourself a little each ride and eventually it will get easier again.

Good for you on your courageous fight back.