I would walk honestly I don't know what I would do as the guy whos sitting, haven't trained. so theres not alot of point in theorising; but here goes in the picture, the attaker is at the side of the seat, where you would be to sit down. so as the defender I would lay on the seat with my feet towards him and pound him, as you do when you're kids. thats a pretty viscious and sustained attack. hopefully don't get cut in the thighs/groin though! if the attaker was in the seat behind, uuuuh.. sit on the floor? (at least out of the way of the knife) maybe the attacker will lean over and allow for a grab and eye gouge? have you got a bag? like a satchel bag? bags are good shields!! hit 'im with it!! that would make a BIG difference. wear a scarf?