Do you know what it is? The foot injury? i would go to physio to get a second opinion. I would also take a total rest from running i.e. not even treadmill - cycling should be good to maintain fitness (i am now out of action with PF and found it doesn't load the foot that much, whereas even walking does).
So now I'm down with plantar fasciitis, what should I do to keep fitness that I gained in the HM training? Went roller skating last night but it turned out painful for the PF affected foot. Swimming should be ok but I'd like to do something else as well. Cycling and general gym seem ok? Circuit clas probably would include some running so I should probably give it a miss. Anything I should avoid in the gym?