Frankly I think it's kinda weird that she's telling you about other dudes already, and while lots of things are possible on her end, I'd say these are the most likely:
1) she's already friendzoned you and will cry on your shoulder if it's there but she'll never date you
2) she's already figuring on keeping you for a rebound guy
3) she's trying to figure out whether you're worth keeping for a rebound guy
4) she likes you but is scared of jumping in too soon after a burn
#s 1-3 are not necessarily total losses, as you learned with Maggie in college, but they have as much chance to be aggravating as awesome.
My advice: Do NOT ask her about this other guy or how she's feeling about that situation, or offer to cheer her up about that specifically -- it will come across as you asking "Aren't you done with this yet?". Listen if she brings it up. If she says she's having a bad day, say you're sorry and ask if you can help. Let stuff happen. And if you lose your patience with all of it, so be it -- at this point you don't owe her anything. Asking if she wanted to go for a walk was a nice low key thing to ask, props on that.
If she challenges you or accuses you about your intentions at some point, say something like "I think you're great and I do like you but I don't want to push you into anything."
But really, it sounds like you're handling things pretty well, dude. Hopefully it turns out well for you.