I'm not putting the weight back on because I'm a different person now. I've learned so much and the new me just doesn't like (and can't physically stomach) the foods and activities that the old me could.
F'rinstance old me would eat bacon sandwiches or fish and chips for lunch every day!

New me would feel sick and not be able to eat for the rest of the day.
Old me used to go out drinking every Friday and Saturday night and go dancing. I never drank much, but I was out late partying. The new me CAN'T stay awake past 10pm because she's so tired out after all the gym time she's put in!
Old me used to "lie-in" till 4pm on a Saturday afternoon. The new me physically can't because she's bursting with energy and is desperate to go out and DO STUFF
Old me used to eat sweets ALL THE TIME. New me ate some sweets and crisps and cakes last night and felt sick until lunch time today!
Old me hated vegetables. I didn't even like carrots until last year! New me has just eaten SusanB's bathtub o' greens! I CRAVED a bowl of veggies for tea, so I had parsnip, carrot, courgette, soya beans, string beans and cauliflower for tea!
Old me used Diet Coke as a crutch to get through the day because she was so tired all the time. New me is only ever tired after a super-hard workout.
Old me thought that sitting in a cinema watching a film was a relaxing way to spend the afternoon. New me finds it very hard to sit still for that long.
On one hand I think that it would be easy to put all the weight back on. I think after a few weeks of eating cwap, you'd get used to eating doughnuts for breakfast and bacon sandwiches and chips for lunch

Once you got over the initial sickness I think you probably
could put the weight back on. But personally I think I'd find participating in the life I have at the moment very very hard without the proper nutrition. I couldn't function at kickboxing if I was weighed down with fish and chips and I couldn't go for long runs without throwing up.
Although I think if I stopped exercising it would be easy to put weight on. Now last year I thought that if I stopped exercising I would never go back to it, so I tried to keep it up even when I probably should've taken a rest. This past month or two I have been struggling with some head games re: eating, exercise and control issues. I've discovered that if I take a week or two off exercise then my body CRAVES to be back in the gym, and it's not just the £30 a month membership that I'm wasting!
Also I'm more aware of nutritional black holes like cheap cakes and biscuits. I know that stuff like that has trans-fats and spikes your insulin BIG TIME. When I was a child my mum always used to explain to me about the sugar crash that I was ALWAYS on! I don't know if I ever really took it in, but then I don't think I had a day without sugar or refined carbs when I was child.
Some more superficial things that mean I won't/can't put the weight back on:
I feel like people are watching me - when I go back home to visit I get the, "Oh you're
STILL skinny..." comments. Well yes. I didn't like being fat all that much.
I don't want to buy a whole new wardrobe! If my trousers are tight, then I workout more and eat less.
I don't want to have to stop doing anything or not be able to do anything because of my SIZE. I remember being on an outward bound course when I was about 13 and I had to get the "extra large" canoe.

I also got the XL wetsuit. I don't want to be last in the hiking group. Coming last in the running group I can handle - I'm still a slow poke but now I can at least run the course rather than walk it!
I don't want to stop running because my joints won't take it, I don't want to stop kickboxing because my fat stops me from being flexible. I don't want to be known as "the fat girl" any more. I don't want to be bullied by insensitive creeps. I don't want to be lusted after by chubby chasers. I don't want people to say, "You've got such a pretty face...". I want to fit in. I want to be light enough to manoeuvre my motorbike without it feeling like a chore. I want to be able to fit into cute clothes in the shops, even if I hate the fashions! I want to be slim enough, so that if we get pregnant I won't look like a beached whale! I want to know enough about nutrition and be in tune with my body so that I won't put on too much weight with a baby. I want to sort my head right out and be able to differentiate between head-hunger and body-hunger so that I don't pass on screwed up messages about food to our future kids.
Ok that was long... And possibly a bit off topic...