of course it is, you could get a non active overweight person to do a full marathon in twelve months. I was due to run Coventry half marathon few weeks back but twisted my ankle in a ditch due to a stupid women driver. Increase training load steadily, start with work at 70-75% maxhr increasing mileage, then throw in runs at 80-85% maxhr (which will obviously be shorter in distance than the low intensity runs) and then bring in some high intensity interval sessions 2 months before.