Originally Posted by
PeteHski
Totally agree with this ^
Reading between the lines I think the OP just needs to do a couple of fairly intensive 1 hour sessions during the week plus one longer steady ride (say 50-60 miles) at the weekend. I think that would be far more effective than 6x 1 hour sessions for the OP's longer event goals. A power meter and focused intervals would potentially give the most bang for the buck with the shorter sessions.
OP is 70 years old. He rides 2-3 time per week for 25-30 miles and you want him to be doing HIIT? Jump right into a 50-60 miler on the weekend? To train for a Century? Why? Are you trying to kill him? Or just train his metabolism in the completely wrong direction for a 7-8 hour century, so he can bonk sooner?
OP is not a racer, a sportif rider, or even a randonneur. He merely lacks the endurance to comfortably ride a century and that is accomplished with building an aerobic base, which emphasises type1 slowtwitch muscles with just a little bit of anaerobic work. He does not need 5 minute VO2 max to bridge up from the team car.