Good work. keep it up. Have you figured out your LTthreshold HR? Reading between the lines it sounds like you're doing your 2x 20's as hard as you can, which actually is not the best way to raise your LT threshold, or improve your time trialing. Figure out your LT threshold, and then do your 2x20's at 95% to 105% of that HR, and you should see your time trialing continue to improve.