The 1874 RLDS edition changed the march to their march in the original phrase “because of the greatness of the march”. The 1908 RLDS edition restored the original the march. Although the 1874 change may be accidental, it is consistent with other usage in the text; that is, elsewhere in the text, we always get a possessive pronoun as the determiner for march (providing there is no postmodifying prepositional phrase):
There are also quite a number of cases where Oliver Cowdery wrote the in place of their (for a list, see under Alma 27:23). Thus the earliest reading in Alma 62:35, the march, could be an error for their march.
Nonetheless, there is a clear contextual difference between Alma 62:35 and all 22 examples of “ march”: all those examples involve a single military unit, either an army or a band of robbers. Here in Alma 62:35, the march refers to the march of two armies, a Nephite one and a Lamanite one. For that reason, their seems less appropriate than the since each army has its own separate march. But since both have marched the same distance, the use of the march works very nicely and avoids the potential confusion of their march referring to the march of only one of the armies. Moreover, a plural like their marches seems strained in this context.
Summary: Retain the original phrase the march in Alma 62:35 since in this passage the text is referring to the separate marches of two armies and thus the definite article the works better than their.