25900

When I saw those repeated five-letter words, and observing that frequency analysis shows the code is not a subsitution cipher as there is no alphabets that are clearly rare (all are about 70), I suspect that it is a vigenere cipher. Then I used Excel to calculate the length of letters between two repected five-letter words, revealing all of which is a multiple of five. This gives furthur evidence that it should be a Vigenere cipher, moreover, with keyword length of 5. By guessing, the keyword is NRICH.
Solution:
I AM ALREADY FAR NORTH OF LONDON AND AS I WALK IN THE STREETS OF PETERS BURGH I FEEL A COLD NORTHERN BREEZE PLAY UPON MY CHEEKS WHICH BRACES MY NERVES AND FILLS ME WITH DELIGHT DO YOU UNDERSTAND THIS FEELING AT HIS BREEZE WHICH HAS TRAVELLED FROM THE REGION STOWARDS WHICH I AM ADVANCING GIVES ME A FORETASTE OF THOSE ICYCLIME SIN SPIRIT EDBY THIS WIND OF PROMISE MY DAYDREAMS BECOME MOREFERVENT AND VIVID I TRY IN VAIN TO BE PERSUADED THAT THE POLE IS THE SEAT OF FROST AND DESOLATION IT EVER PRESENTS ITSELF TO MY IMAGINATION AS THE REGION OF BEAUTY AND DELIGHT THERE MARGARET THE SUN IS FOREVER VISIBLE ITS BROAD DISK JUST SKIRTING THE HORIZON AND DIFFUSING APERPETUAL SPLENDOUR THERE FOR WITH YOUR LEAVEMYS