I think you should find a good route to race on (probably where you do your 20 min tempo, but longer), and go out and try to do at least a couple 20 min TT efforts (and maybe some 30 or 40min ones, if you have the days available) to get an idea for pacing and how you will feel. The goal will be to go as hard as you can such that you are barely able to finish the interval. Keep an eye on whatever data you use: speed, cadence, heart rate, power, whatever. Just know that if you use HR, it can be more variable than power. What you're going for is tuning into a "feel." But know some numbers so that when you get out there, if you're perceived exertion is skewed for whatever reason, your data can help you convince yourself to go harder when you think you're going hard enough but aren't. After a few of those practice intervals, you start to get what you can sustain for how long. This will get you through the 20km TTs. 40km is a much tougher game for me.