Yep, interval training will help. Without it, you'll plateau, and while you can ride longer, you won't ride those distances faster. A well-designed training program will also help you avoid burning out from doing super-long rides for weeks (months?) on end without variation.
Although I'm quite confident in my opinions
and this is not PBP, if you are thinking competitive you may want to hire a trainer and get professional advice.
Here's an NYT article on interval training:
http://www.nytimes.com/2007/05/03/fa...03Fitness.html
And another link:
http://www.ultracycling.com/training/peaking_plan.html