Hi Anna,
well done on your progress so far. Like Katie suggested I to would be very cautious about taking on too much. Excercise, especially running, is very addictive and it is very easy to get carried away and not look after yourself as you should.
Getting advice from a dietician or nutritionist is a very good idea. I would also reccommend weighing yourself regularly, maybe once or twice a week, just to make sure you arent loosing weight. If you find it hard to stomach eating more food, there are plenty of alternatives on the market. Most health food shops have a wide variety of protein/carb drinks which you can have between meals, just to ensure you are getting the nutrients you need. There are lots of good running books on the market, some have meal plans which are very helpful if you struggle with knowing what to eat, and how much etc.
Bear in mind you can burn around 100 calories per mile, so you will need replace this just to maintain. Also it can be very tempting to skip meals on the days you dont excercise, you try to convince yourself that you dont need to eat as much as when you do run. Be very wary of this.
I suffered with an eating disorder in my late teens and have tried on numerous occassions to get back into excersing but old habits die hard and It is all too easy to slip back into your old ways. It has taken me 10 years to get to a point where i can excercise sensibley and know my limits, and its been very hard. You should definitely discuss this with your GP before even thinking about applying for a marathon.
Good luck with everything