3cannondales, training to increase one's speed for a century is going to require some resturcturing on your current regimen.

Terrymorse is spot on about increasing one's speed. It's all about MAP (Maximum Aerobic Power) and increasing one's LT (Lactate Threshold). Those just cannot be trained efficiently via 80 mile rides. I would suggest you do what he recommended but in addition only have 1 longer ride per week which could be about 40 miles at up to Tempo pace or just below. Cut back on your volume and save your energy for those Tempo rides and LT intervals. Don't go out and blow yourself up doing more 80 milers. You could limit those to once a month but don't try and keep your speed work up on those weeks.

In addition to that I would say you can do recovery rides around 65% - 70% of MaxHR but they have to be brief! 30 minutes. No 2 hours at that pace as that just became an endurance ride.