Its a short walk, around 10-15 minutes. Being an away fan you will have go to the far side of the ground but thats not hard as Fratton Park is quite a small ground.
If you want food get it outside the ground the catering in Fratton Park (like most grounds) is pretty bad, there are plenty of burger vans around but I would recommend Micks Monster Burgers.
PUP
Pompey Paul