Just wondering as sometimes people may offer slightly bad advice on HR, Cadence etc without taking age into it. We are not all spring chickens 😆
I am like you, averaged 29 mph for 1 1/2 hours and my knackers were dead after 50 minutes. I have lowered the saddle 1/2" and pointed the nose up a little as well as having a small block of wood under the front wheel.
This has worked for me before so it may help you (worth a try).
I also purchased a portable dvd player as my trainer is in the garage so this helps the time go by and gives me some peace to watch what I want to watch for once.
Sounds like a well structured interval session lasting no more than 40 minutes will do what you need.
Mix it up, keep it fresh and throw in some climbs.
But what you need to ask yourself is, "Am I doing this to get fit or just to get some exercise"? As an inerval session is a lot of pain to go through just to have a bit of exercise.