How long is your small window of time?
If you have say 40 minutes and it is a nice day, I would think of walking to the Penn Campus area where there are always lots of food trucks. That might involve a 10-15 minute walk. I don't know exactly where the trucks are but I think approximately around 33rd and 34th St and Walnut.
There are food vendors right in 30th St but I don't know if there is a cheesesteak place in there. There is also an Irish Pub, Slainte, on Market St across from the station. It might be possible to get a cheesesteak in there but probably would not be as authentic as one from a food truck.
Honestly if you don't eat meat I am not sure it is worth it!