I don't think that you can necessarily make that assumption
You're intuition serves umd, I'm only in my third year of real racing/training and I only really started doing 3+ hour rides consistently last year, if there's one thing I'm constantly looking to work on it's my aerobic engine. I've never had problems with sprints, 1 minute, or 5 minute drills, it's the long term efforts where I'm constantly working.
Slim_77, since I'm racing collegiate the only thing I pay for is food on the weekends. I'm more focused on racing for the betterment of the team (ie points for nationals) rather than sitting in to test fitness or what not.
Kensuf, the rest idea is definitely a good one. I'll really have to play things by ear these first few weeks, I had a major case of burnout by August of last year.