While you may be planning to lose 12 kilos in 6 weeks, that's not considered safe weight loss, as recommended, both on this forum and by most medical experts. Safe weight loss is considered to be 1-2 pounds/week, accomplished by lifestyle changes that support and help maintain the loss.
Many of us have medical issues that make losing that much weight in so short a time, impossible and in some cases, dangerous.
As long as you enough carbs before you run, ie. Porrige or a jacket potato an hour before you run and have a protein shake to hand for when you finish, you'll be fine. I know huuuuge guys who do loads of cardio and maintain size. Diet and hydration is key.
Hi there I am Jay I do heavy weight training 5 times a week is it OK for me to do cardiovascular 3 times a week or will I lose all the muscle that I have gained any help you can give me is good n helpful thanks.