It's bloody easy to get into bad habits - I have done many times in the past - my DC have had their fair share of junk and lazy days too..... I work full time and sometimes take the easy option just for a bit of peace or to catch up with myself... Marshmallow fluff and pop tarts being obvious examples 
At the moment I am just getting my 12 yo DD to take up running - doing a 4k circuit with me 3 times a week running and walking to get her started (I am training for a half and have lost 2 stone since January with 5:2 diet) as well as trying to overhaul our family eating - reducing refined carbs especially sugar, increasing fruit and veg, and generally trying to improve the quality of food whilst watching portion sizes / quantity, cutting down the amount of alcohol I drink, encouraging the DCs to drink a lot more water than squash or diet fizzy....
I agree with the other posts - you need to take charge especially of her diet, but be prepared there will be some bad times to start with.
Aim to cut out junk for your meals and make a point of all the family eating the same - healthy food cooked from scratch, and pay attention to portion sizes - 6 year olds are growing but still don't need a full adult sized portion usually... If possible get her to help plan, shop and cook healthy stuff - maybe choose a new healthy recipe book and put her "in charge" of selecting meals for you all to try....
Also cut out most snacks, have a jug of water in the fridge for drinks BUT have a designated day when she can still have some sweets or an ice cream - do this once or twice a week rather than every day so it is a treat, and if you have family stuff planned, take a picnic rather than McDs - have you looked at how many calories are in MCDs meals? happy meals are smaller, but a standard medium meal ranges from around 750 to well over 1,000 depending on diet v full fat coke etc...
In terms of exercise, I'd look at overall activity levels first.... turn off the tv and get outside during the holidays - get her walking, cycling, scooting much more - don't use a car or the bus if you really don't have too - its amazing how far 6 yo's can walk with encouragement - at that age mine were doing 12 - 13 mile hikes at weekends in the lake district or peak district!
Then go to the local pool as often as you can and encourage her to be active but be careful about after swimming snacks...
And talk to her to find out if there is something she'd like to do - eg roller blading or maybe karate or tae kwondo or similar? or street dance, ballet or gymnastics? encourage as much as you can - and think if there is something you could do together - tennis, go bowling, ice skating....