The purpose of a warm up is to prepare the body and mind for the session ahead. The key factors to this are increasing: the heart rate, blood flow and body temperature. It is important that the warm up is progressive and dynamic. The most simple way to warm up is to start by walking, move into a jog gradually increasing pace then finish with some strides (faster paced running). Static stretching is NOT recommended as part of any warm up routine unless of course there has been a particular injury and a specific muscle/muscle group needs to be prepared. Although a warm up can include running drills , it can be better to focus on drills as a separate unit in a training session plan. Running drills can include the following exercises:

Allow a minimum of 10 minutes for a warm up, in advance of a specific training session, especially high intensity training then this can be extended to 15-20 minutes. The routine can and should be amended to support the specific training session being undertaken. Failure to perform a warm up prior to exercise may lead to injury. Excessive warm up may reduce the pace at which you perform in the specific training session.
In advance of a long training run using the first few miles running at slower pace is acceptable as a warm up.
Prior to race a below half marathon distance a warm up is advised and finishing with strides can be beneficial.