Here's another way to think about it.

Say you need to move 1,000 bricks from one end of a yard to another. How many should you carry at a time to get done the fastest? What's the most efficient load?

Think of one brick per load as your lowest gear, and go up from there. You can put one brick in each hand and run back and forth (high cadence), or you can take bigger loads and move slower (low cadence).

How many would you move per load if you were moving only 20 bricks? What if it was 2,000?

You might be able to start out carrying 10 or more bricks at a time, but how long can you maintain that? And will it tire you so much from the outset that any gains you get will be offset by how much more tired you'll get?

What if you moved bricks every day? As you got stronger you would be able to handle bigger loads as well as move faster with those loads... (Roody's point)

What if there was a hill in the yard? You have to walk on a flat section until you get to the base of the hill. Would it make sense to carry a bigger load (use a bigger gear) on the flat part, then unload a few bricks at the base of the hill (drop to a lower gear), and carry them up separately?

Choosing the "right" gear is a lot like choosing the "right" brick load in these examples. The answer depends...