It is best to have done a full preparation for an event but when there is insufficient time to do this you should bear several things in mind. First of all don't try to cram more running to try and make up for lost time. The objective is to reach your optimum state of fitness on the day itself and then run to that level of fitness. How far your progress will depend on your individual state of fitness. Don't increase training unless you are coping with what you are doing.