One hard interval session riding 6-10 minute intervals as hard as possible, as many as you can complete remaining above FTP. That intensity range will have the highest impact on VO2max and FTP. Do this after a rest day so you're as fresh as possible. You can do this on hills or flat ground, although many people have an easier time pushing themselves on hills.

Your group ride for fun.

Otherwise below your aerobic threshold (where conversation doesn't flow, breathing becomes rhythmic, lactate starts to accumulate, about what you can manage with an even power split on a five hour ride) the rest of the time. This is usually in Friel's heart rate Z2. Use low enough gears to maintain an acceptable cadence (this may mean a mountain triple for some riders) or avoid hills.

Make one day a long ride day and go twice as far as you usually do.

The endurance pace will make you good at going slow, which can reach 20 MPH all day every day on flat ground for a larger fit rider. It'll also run primarily off fat, so you can eat less without hunger and reduce fat until your abs are visible (about 10% body fat for men) so you're not hauling more weight up hills than you need to.

This works on 6 hours a week, but you want to ride more - a lot of people (Greg LeMond, Connie Carpenter, various coaches, random internet people) think 10 hours a week is where things come together and you achieve most of your genetic potential. Work up to that adding 10% to your volume each non rest week.

Six days a week is good.

Take a lower volume, low intensity rest week out of every 4. That's when adaptation occurs.