This is a battle mostly between rookie Montee Ball and second-year back Ronnie Hillman.
As you can see in the above video, Hillman likes his chances and has been working hard to be in position to be "the guy" in the Mile High City.
Interestingly enough, it's not as insane as it sounds.
ESPN Denver radio personality and NFL writer Cecil Lammey has been embedded out at Dove Valley for all of camp so far (check out his Google Hangouts from training camp). If you're a Broncos fan, you should read his stuff both at 102.3 ESPN and on the Footballguys message boards, where he shares all sorts of tidbits during and just after camp.
Lammey has some great takes on not just Ball and Hillman, but Knowshon Moreno as well.
Almost the first thing he does in the thread is remind us that John Fox doesn't tend to favor rookies in position battles. Now, the Broncos lack a running back like Fox had in Carolina, but it's something to keep in mind.
He also reminds readers that with the Broncos, "starter" is a relative term. It's "ceremonial," he says, and just because you're the starter doesn't mean the offense will use you exclusively. Fox and company will go with the hot hand, be it Hillman, Ball or even Moreno.
He also said that he feels Ball will get the red-zone and goal-line looks regardless of who is the starter, but also that Hillman looked better in general during Day 1 practices.
He wanted to see Ball with the pads on, but unfortunately, when that happened, it wasn't what anyone had hoped for.
Lammey also said that Hillman was getting the first-team reps while Moreno was getting second-team, and at one point Ball got no reps.
In general, it sounded like as much as Ball failed to meet expectations, Hillman exceeded them.
It could be that Ball is struggling the way any rookie does, while Hillman is doing well because he has a year under his belt. That's how things go at times, and at some point the light will probably go on for Ball.
It really seems as if Hillman has a lead on Ball as of now. As Lammey says on the message board, "[Hillman's] created a gap that Ball is not closing at this time."
That said, he reiterates that this will be a running-back-by-committee approach no matter what, and he expects Ball to end up as the "go-to guy" at the goal line.