bigfred 

Think about the purpose of each workout and/or block of workouts.

If performed properly, High Intensity Intervals should leave you spent. Even the longish ones you're probably doing as part of the new century TCCP. By doing 'more' you're just adding to the fatigue your muscles have experienced and hindering their ability to recover for the next workout.

Time better spent would be if you used that 'extra hour' per week to read your copy of The Cyclist's Training Bible:-)

That would help you understand where the time investment break points are for yourself and help you transition your training from the 6-9 hours a week that TCCP is based on to the 10+ hours per week where a more traditional Friel'esque plan pays dividends.

Then, you'll be able to actually 'plan' for optimizing the use of 90 minutes, twice per week, instead of making ad hoc adjustments.

The exception to this might be if you were using those 30 extra minutes for very light load, high foot speed, cadence work. But, I would probably put such drills at the beginning of the workout, as part of an extended warmup, preceeding any HII.