You can probably finish a half marathon after you've run 10 miles as a long run, but if you want to finish the half feeling great and with low risk of injury, I suggest you should do long runs of 14-15 miles a number of times so your body can partially adjust to the stress of the 13.1 distance. Think of a half marathon (or full marathon) in terms of stress on your body rather than in terms of miles. Your question then becomes what can I do to reduce the stress of the half marathon so I feel great at the end and have little risk of injury. Of course the answer to that question has to be to let your body experience that stress and adjust to it. This means that you must run at least 13.1 miles if your body is to adjust to the stress from that distance. If you only run 10 or 11 miles before the race, your body will experience the stress of a half marathon for the first time during the race, and that may not be a pleasant experience for you.