Earlier quoted context omitted.
We're talking about soldiers stalking the wilderness of Pliny the Elder's past, not the present-day United States where game populations have declined dramatically. Furthermore, the population figures are way out of whack as well. The city of Rome in early imperial times was at best half a million people. Pliny the Elder's hometown of Como in northern Italy might have housed up to 10,000. An army drawn from that city…
2000 years ago in the regions where the Roman army operated, animal husbandry was already an established way of life for 5000 years or even (in some spots like Asia Minor) a lot longer than that, and the overwhelming majority of the mammalian biomass was in the form of domesticated animals, not wildlife. The land that was not under cultivation was either quite hilly or had something wrong with it that make it bad for…
The article I listed explained in detail how Roman soldiers carried out the full process of turning grain into flour and then baking bread in their encampments. You don't think they could have managed the slaughtering of livestock?
But besides that, there were plenty of forests around (which they used to gather firewood, as mentioned in the article). Those forests absolutely would have contained deer and other game they could hunt and preserve.