Ok you do not have a leghorn, you have a Cornish Cross. Leghorns are slim birds that are one of the top laying breeds. Yes she can be made into a layer but it takes some effort on your part and accommodations. Fist and foremost you must limit her feed. Put the food in a raised location, somewhere out of your CX reach. To keep my meat birds healthy I only feed them first thing in the morning and last thing at night whatever they can eat in 15 minutes. Many only feed them at night so that they are better encouraged to forage.
If she is freeranged she is much more likely to live a longer life. The more this bird is made to move the better.
She is not going to be able to fly and be cautious of any falls. This bird falls like a brick and can break a leg. Build ramps and set things up for ground roosting.
There are many who have experience raising these birds to laying age, it is possible, but be aware it is also likely she will die early. Many have heart attacks and other health problems, they were not made to have long lives.
Edited by OrganicFarmWife - 5/24/16 at 8:13pm