Heck, this might even be Plan A.
In an earlier post, I had wondered if there was any best option other than to run for the exit given the OP's parameters. I hadn't considered someone severely limited in their mobility. In that case, perhaps getting into that storefront 5 feet away makes more sense than going for the exit 30 feet away. That's certainly a mitigating factor that can make running away NOT the best option.
I think hunkering down (with or without a weapon) will be Plan A for most people in many active shooter situations, whether they want to or not. As I mentioned earlier, I think the fact that we can immediately recognize the shooter, their location, that they're doing something bad, that the exit is close by, is a huge gimme in this thought exercise.
I presume that for the bulk of people actually caught up in such a situation in something like an office building, they will be away from the immediate action and won't have any direct information to go on. They'll hear a pop-pop, maybe even muffled by the walls, and ask "Did you hear that? What was that?" Pop. "Is that...?" Pop. Scream. "What? Where is this? What's going on?!" Maybe you'll see someone run one way--but then someone else run the other way. "What the HECK is going on?!"
In this more realistic scenario, immediately blindly running towards a distant exit may not be the smartest thing if you don't even have an idea where the shooting is occuring. (Off topic, but with the 9/11 anniversary coming up, everyone blinding heading for the fire exits at the same time in a very large skyscraper is not necessarily the best course of action either, what happened on 9/11 notwithstanding).
In this case, holing up/hiding, at least until you get more information, could certainly be considered a better option depending on the situation.