OK. Well, Hitchin isn't that big and you could walk into town within 10 minutes for somewhere like Pizza Express, which is at the top of Sun Street and Market Place if you have a bit more time.
If you have slightly less time, you can walk from Benslow Lane to Hermitage Road which has a pretty standard cafe called the Hitchin Cafe, the kind of place you could get a jacket potato etc and be done within half an hour.
Even less time and apparently there is somewhere on the Walsworth Road between Benslow Lane and the station called Hitchin Kitchen, I've walked past it I think and it is a very small cafe but I imagine it would do hot food. The Radcliffe Arms doesn't seem to do food at about 3-4pm.
It's 10 minutes at most to walk from Benslow Lane to Hermitage Road, and maybe 20 minutes back? You might be able to get a taxi but as Hitchin is small the taxis tend to be in ranks it would take you just as long to walk to and then cab to the station as it would to walk to the station.
If you do feel close for time i.e. appt finishes at 3.30, I'd say it is safest all around to walk back to the station and go to the McDonalds opposite (it's sort of tucked around the corner so you might need to ask someone, but it'll be on Google Maps) - it's not tempting but is probably the safest for time.