2x20 has it's usage but for 2 hr TTs then those lower L4 upper L3 intervals that last 1.5+hr are great for this. Plus as you'll find out soon that holding a high intensity for 2 hr straight is mentally quite different from 20 min intervals. Plus doing all those long duration interval will really help your complete fitness in many ways. See the "SST to that PMC" thread.
Greg