Training & Nutrition - Improving Metric & Century Times

I'm looking for some opinions on improving times for longer-distance events like centuries. I've averaged about 2-3 centuries per year for the last 3 years, plus ~80-milers most spring/summer weekends, so now I'd like to start working a bit more on speed. My normal century time is around 5:40, but I've done 70 miles in just over 3:30 (on a good day), so a 5-hour century may be within my grasp.

I'm thinking of doing some workouts later this spring where I use a normal pace for the first ~3 hours, then do some intervals and/or hill repeats of increasing intensity for the last hour or two.

Would there be any benefit to doing this? Or am I better off sticking to intervals on my shorter, interval days (maybe increasing the duration of TT intervals) and staying mainly aerobic on my distance days?

My best century times are a lot slower than yours (hilly routes though, it's those hills, yeah that's it) so take this with a grain of salt. A couple of years ago, I saw a "fast century" training plan on (don't laugh) Bicycling Magazine's site. The plan used different types of high intensity intervals on shorter rides during the week; long rides on weekends were done "at pace" (aerobic), with a few intervals spaced within. I thought the program was consistent with what I've read from Friel, such as his century training plans in "Cycling Past 50".

Hope this helps.

I've got Friel's Training Bible, but I might see about getting the 50+ book, too. From what you've said, though, I gather more than one source recommends adding intervals to longer rides.