Use your base as the foundation for some high intensity work. Intervals; short, medium and long. Race scenario drills. I wouldn't do too many thirty minute race emulations. That'll chew you up pretty fast. Instead one day a week do a set of four ten minute intervals. That way each one can be greater than race intensity and you get even more training and less stress. Find some slog, hills or wet grass or whatever your local terrain offers to steal your momentum and make you suffer. Try to do shorter intervals on that with an emphasis on keeping the snap in your pedal stroke. Do that maybe once a week. Combine some mounts and dismounts with these, add in a recovery ride and maybe an easy distance ride and you've got a full week in relatively few hours on the bike.

I'm not at all an expert, but this is about what i'm setting out to do in my limited training time and I do believe it's working.

Ron