The phrases "flew by" and "like one solid wall" suggest that Searchlight and Little Willy were moving at a very high speed and in a relatively confined space. This can be supported by the following details from the story:
Earlier in the paragraph, it is mentioned that Little Willy was "leaning forward as far as he dared" while riding Searchlight. This suggests that they were moving at a fast pace and that Little Willy was trying to maintain his balance.
In an earlier part of the story, it is mentioned that Little Willy and Searchlight were practicing for the race by running through a narrow path in the woods. The phrase "like one solid wall" reinforces the idea that the trees were very close together, making it feel like a confined space.
Therefore, the phrases "flew by" and "like one solid wall" suggest that Searchlight and Little Willy were moving at a high speed in a narrow and confined space.