Based on the last sentence, what is King going to do with the letter?

From the text, we can infer that he is going to mail the letter to the clergy is was meant for.

Never before have I written a letter this long -- or should I say a book? I'm afraid that it is much too long to take your precious time. I can assure you that it would have been much shorter if I had been writing from a comfortable desk, but what else is there to do when you are alone for days in the dull monotony of a narrow jail cell other than write long letters, think strange thoughts, and pray long prayers?