"What I do say is that I can write verse, and that the writing of verse in strict form is the best possible training for writing good prose"

Philip Pullman is suggesting that composing verse in a strict form is the best way to train oneself to compose good prose. He thinks that composing verse is a skill that can be established and honed, and that this skill can be moved to composing great prose. He is highlighting the significance of structure and type in composing, and how this can assist to produce a strong foundation for composing excellent prose. He is likewise recommending that composing verse can help to enhance one's writing style, as it motivates the author to think more carefully about the words they utilize and how they are set up. By composing verse in a strict type, the author can learn to be more exact and concise in their writing, which can then be used to composing excellent prose.