Legatus Sollemnis arrived on the Wall at the Rock with the Sixth Legion’s cavalry detachment shortly after darkness fell two nights later.
Problematic sentence (follows the previous one):
The rest of the legion was more than thirty miles back down the road to Yew Grove, encamped after a day slogging their way north at the forced march, and still a day away.
Could someone kindly give me the explanation of how come that in one sentence, the Subject "the rest of the legion" agrees with both the singular "was" and plural "their"? Why it's not "slogging its way"?
Also, reading the "after a day slogging their way north at the forced march" part I had the feeling that the preposition "of" before "slogging" is missing. Should it be "after a day slogging north" (like "a walk during the whole day") it wouldn't draw my attention, but the pronoun "their" to me personally made that part read somewhat oddly. Please explain to me why there is no preposition there.