Tom you don't need to do long rides to do very well in longer races. The key is to build your threshold power as high as you can and build a good VO2 max.
Fitness is a byproduct of plenty power at threshold and a good VO2 max. The things that contribute to this the most are mitochondria density, good vascularization to the mitichondria and a strong cardiac output.....all these elements are built through high intensity efforts WAY WAY better than by endurance riding.
The endurance "base" ideation is a myth....a tradition...a habit. That doesn't mean there is no place for longer rides...there is...but once you are in half decent shape the role of endurance rides is mainly to teach the body to store large amounts of gylcogen, to burn body fat and to teach you to handle sitting on the bike for long periods of time.
The whole goal of the base period is to build threshold power.
In fact 20 hours a week at endurance intensity can actually be quite detrimental to the average guy that has a job, wife/girlfriend/kids etc etc. It will slowly drain you and eat away at recuperative powers and lower testosterone...and prevent you from doing much threshold work or even tempo work.
Guys like Dr. Coggan and Greg Lemond know all the above so you will be in good company. Dr Coggan has a 300FTP at about 65 kilo's, 48 years of age...and his busy life allows for about 5 hours a week on the bike!...but smart hours.
Please read these articles.
http://www.biketechreview.com/performance/base.htm
http://www.biketechreview.com/performance/mitochondria.htm
and the practical application of these concepts:
http://www.biketechreview.com/op_ed/stripped_down.htm
http://www.biketechreview.com/op_ed/stripped_down_2.htm
You have only 6 hours a week to train...spend almost all of it at tempo pace and up...save for warm ups, cools downs and some endurance work once in a while ...and some in the group ride.
Tom I think you should try the program I outlined for you.... do the those three day blocks...then take two days REALLY EASY and then try to get to get in a tough 3 hour group ride with those guys on Saturdays if you can...and then rest on Sundays before repeating the block. Train hard...rest just as hard.
The three days of recovery can be totally off the bike if you like. Perhaps you can go for a walk with your wife or girl friend.
Start the VO2 work now or in a couple weeks (sets of 5 minutes at about 113-115%FTP, with 5 minutes easy between). Drop the VO2 work, most the time, in the off season and focus on threshold work(95-105% FTP, mosty about 95% and some 98-105% as races approach), "sweat spot" rides(88-94% of FTP) and tempo rides( 76-90% of FTP).
You will be amazed at how much you can actually get done in 6 hours a week and how much you will progress.
At your age you CAN build your FTP to at least 4 watts per kilo with 6 hours of training a week...do that and you do very well in masters races at 35-40.
I would HIGHLY suggest that you buy a Kurt Kinetic trainer with it's power meter so some of your work. Make sure to blow a good fan on you as you train with it. This machine is the best trainer under $1800 US BY FAR and allows you to do the tough stuff by power.
To find out your FTP do a 20 minute all out effort on this trainer....and if you have average anaerobic work capacity like most guys then 95% of the average power will be pretty close to 100% FTP for you.
Trainer work makes you very strong for many reasons but mainly because the lack of inertial loading and pressure on the legs through a longer throw of the crank arm causes is unrelenting....it makes the road feel easy actually.
Plus the environment is totally controlled and repeatable...great for threshold and VO2 work.
http://www.kurtkinetic.com/
Then later you'll want to get a Power Tap for outside rides.
Good luck and keep us posted!