Where could you? Where should you? Where do you have no choice?
You need to start being fast. As you master the necessary technique, you have to remember to make it pay off; don’t forget that in the beginning (when you are practicing) you aren’t reaping the benefits (you are limiting the losses [which will only become a kind of benefit over time.])
You need to start saving more (true) but also looking for places to spend what you save (don’t spend it but ask yourself “where could I [where should I,]”) like when you creep (or sneak really [steal in a way]) into a really big gear and you’re already going fast without having done anything to get there.
Where are the small obstacles (that get smaller as the speeds increase [that will provide ever larger gains]) given their position in relation to what helped build your speed (descent, tailwind, match, etc.)?
It might be worth it to spend your energy holding that giant gear over a small rise or into a gust of wind (probably always with a pedaling technique that waits [except when it is now or never (and it rarely is)] but, all the while, not fighting back against anything that isn’t worth the tax.
You have to limit the damage, between 40 and 60 kph (that’s where it matters [unless you’re on a mountain, and you aren’t] in terms of average speed ; ) forget about 20 to 30.
The technique of sitting back to get to speed and the ability to sit back after a match are the same. By combining proper pedaling with the decision of where one could build speed with a match (as opposed to waiting patiently for external forces to diminish) and the choice of where to use that match (as opposed to profiting strictly from the external “help,”) you won’t have to work to get up to speed (especially not in the tailwind [or on a descent.])