I really like It Seemed Like Thin Air (Mash-Up) which will take you 2:28 hours to complete. It’s a nice workout and to quote the official description:
Who needs the alps when you have this abomination of climbing? If you’re looking to churn out enough tempo work to make the domestiques at team sky look twice this is the one for you. Believe it or not, packing so much tempo into 2.5 hours gives this session the same endurance building power as a 5 hour “Zone 2” ride. That being said, tempo efforts burn through more carbohydrates than endurance pace, so don’t forget to eat during this epic ascent.
If this workout length is already stretching your limit a bit too much, you can always go with Blender (1:42 hours) and get a nice workout done.
If you feel up for it and are on a smart trainer; switch to level mode, ride on rpe and give it all you’ve got.