I too have a goal for September, to run the great north run. I am 5"8 and 11 stone 8lbs and can run about 3 miles at the moment so I am just going to do my best on the day. But a big part of the journey for me is to lose weight in the process.
I have just got all the food in for 'anna richardsons body blitz diet' which i picked up from the library. Its realistic, has simple rules and loads of recipes of home cooked food.
It is definitely realistic for you to loose over a stone in the next 2 months in time for your holiday. But your exercise will make you feel hungry.
Cut out bread completely (flatter stomach instantly), limit your dairy and try to eat no carbs after 6. Did this last year and the weight dropped off and all the chicken i ate meant that i was never hungry. Back to my diet tomorrow.
You will definitely do it with a positive frame of mind and determination.. good luck!