mjhjmtu

Zugvi gsrh ormv, ivzw gsv mvcg gszg rh lww,

Evib jfrxpob, svzw gl gsv ormv gszg nvzmh gsv kivurc jfzw,

Tllw! Blf'iv xzgxsrmt lm, uiln sviv gl ormv hrc,

Vevmgfzoob blf'oo urmw gsv nrcvw zxilhgrxh.

R gsrmp blf hslfow xlmgrmfv fk gl ormv gdl zmw blfi qlfimvb'h lm rgh dzb,

Tllw, xlmgrmfv drgs gsv nlmgs drgs z xlhgfnvw wzb,

Vevib hrmtov ivnzrmrmt xfyv rh dszg xlnvh mvcg,

Hdrugob tl gsv gsv hjfziv lu gsv hvxlmw ormv ivzw lu gvcg,

Hzfmgvi luu gl mfnyvi urev, xlnv lm, blf'iv wlrmt urmv,

Irtsg! Mld tl gl gsv illg lu uligb-mrmv,

I think the solution is

AGGRESSIVE

Because

Doing the substitution as per QuantumTwinkie we get a poem.
Reading the lines in order 1,3,6,10,7,8,9,5,2,4 we take the first letters of each line (hidden acrostics), end of line 4, we get the word 'AGGRESSIVE'.

Edit:

The lines once substituted:


1. After this line, read the next that is odd, Go to 3

2. Very quickly, head to the line that means the prefix quad, Go to 4

3. Good! You're catching on, from here to line six, Go to 6

4. Eventually you'll find the mixed acrostics. The end.

5. I think you should continue up to line two and your journey's on its way, Go to 2

6. Good, continue with the month with a costumed day, Go to 10 (Hallowe'en)

7. Every single remaining cube is what comes next, Go to 8 ($2^3$)

8. Swiftly go the the square of the second line read of text, Go to 9 ($3^2$)

9. Saunter off to number five, come on, you're doing fine, Go to 5

10. Right! Now go to the root of forty-nine Go to 7 ($sqrt49$)

I found

After putting it into a substitution solver I got

This

After thi? line, read the ne?t that i? odd, Very ?ui??ly, head to the line that mean? the ?refi? ?uad, Good! You're ?at?hing on, from here to line ?i?, Eventually you'll find the mi?ed a?ro?ti??. I thin? you ?hould ?ontinue u? to line t?o and your ?ourney'? on it? ?ay, Good, ?ontinue ?ith the month ?ith a ?o?tumed day, Every ?ingle remaining ?u?e i? ?hat ?ome? ne?t, ??iftly go the the ??uare of the ?e?ond line read of te?t, ?aunter off to num?er five, ?ome on, you're doing fine, Right! No? go to the root of forty-nine,

Maybe I can fill in some blanks to get

After this line, read the next that is odd, Very ?ui??ly, head to the line that means the prefix quad, Good! You're catching on, from here to line six, Eventually you'll find the mi?ed a?ro?ti??. I think you should continue up to line two and your journey's on its day, Good, continue with the month with a costumed day, Every single remaining ?u?e i? that comes next, swiftly go the the square of the beyond line read of text, haunter off to number five, come on, you're doing fine, Right! No? go to the root of forty-nine,

So

The root of 49 is seven?