Actually if you can do 2 hours then the 2 hour SS/Tempo sessions are probably the way to go - something like Round Bald or Spruce knob but the +2 or +3 versions - they are only at 85% but they give you a longer workout. Most of the people I know who post on the UK time trial forum reckon that time in zone trumps intensity so 4x20 @85% is better than 3x20 @90% - also I find the 2 hour SS sessions are less boring than the endurance…the pain keeps you occupied - if you can work up to doing Wright Peak or the -1 version you know your in good shape. Shame you can’t get outside - I did 4 hours zone 2 in the sun this am - lovely day, solo ride, me and my thoughts…mental health boosted! 
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