Im in late 40's as well. 30 minutes isnt a huge deal but the ops trudges along for 60 minutes a day to burn what 260 calories? Whats the purpose? Whats the goal? There is virtually no benefit from day in and day out steady state, slow trudge cardio beyond burning that 260 calories.
Your 15 minutes of HIIT is all you really need based on your above comments, that 15 minute steady state likely isnt doing anything for you but burning about 60 calories. YOU have a goal and are doing cardio to meet your goal, although I think you could cut out the steady state part.
Your equating your 15 minutes HIIT and 15 minutes high output to the ops slow steady state and its not even close to the same thing. What you are doing and what the op is doing isnt even comparable.
The op is complaining about the need to burn calories but then using a time consuming poor method to do so. He could burn a lot more calories in a fraction of the time AND at the same time improve VO2 max even without running.
My "attack" is not on your goal oriented approach its on your comment that steady state cardio is "very effective" because its not, not when I can burn 2-6 times more calories AND improve endurance in less than 15 minutes.