Hit this link to take you to an article posted by Jack Daniels on this topic.
Research shows you shouldn’t be too worried about losing significant fitness if your break from running is less than two weeks.
You’ll lose some conditioning in your aerobic system and muscles, but pre-inactivity fitness will return quickly. Again, this assumes that you have built a healthy and consistent base of training of 4-6 months prior to taking time off. It’s not the end of your career if you haven’t been training for this long; it simply means that the reduction in fitness will be slightly more pronounced.
After two weeks of not training, significant reductions in fitness begin to occur and you’ll have about 2-8 weeks of training (depending on the length of inactivity) ahead of you to get back to your previous level of fitness.