Riding a long TT you're at or slightly above Lactate Threshold for a sustained effort. Thus to go fast, you need to train to 1) be able to hold an effort at LT for the length of the event , and to raise power at LT. One of the best way to do this is steady state intervals. Steady states are longer efforts at LT. For a 40k TT you need to work at LT for approximately an hour. So when I'm preparing for the state TT championship (the only 40k TT I do) I do steady states working up to a total of an hour. Steady states should be at least 6 minutes, and done on 5 minutes rest. So you might start with a set of 4, 6 minute intervals, and work up to 6x10, and 3 x20.
If you don't know your LTHR, there are threads on that.
You can obviously do other intervals, and need to mix things up with rest and base miles, but IMHO, Steady states should be the core of your TT training.