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?