It is somewhat more realistic if you will be going from a standard road bike to a good TT setup, but 5 minutes is still a lot to ask for and will likely take multiple years of training, unless you really bombed your 1st effort. It is good to have goals, I typically set 5 or so attainable goals specific for every season, anything beyond a season while great to think about is more of a dream than a goal. My suggestion was to merely look at the 5 minute goal in more detail and assess if it is a realistic goal or more of a dream, if it falls in the dream category I would set a goal that is more attainable.
For perspective I did our local club short course TT (9 miles) in 22' 30" back in April using my road bike with aero bars, a couple of weeks ago I did the same course with the same setup in 21' 30", shaving a minute off of my previous time. While I am somewhat aero my setup is not great (I am reaching quite a bit and have difficulty shifting on the rollers). So next year I will be throwing some actual TT bars on my backup road bike and hope to shave time that way, this coupled with an aggressive training regimen in the off-season I hope to break 20' 30" next season on the same course. Sure I really want to break the course record that is somewhere around 19' flat, but setting that as a goal for next year is not realistic.
Again your blog is a neat idea, especially if you keep up with it throughout your progression, I just noticed the 5 minute goal and figured I would try to help with a bit of perspective.
Best of luck to ya!