You're not going to suddenly lose fitness because you missed a workout! Keep in mind that you're supposed to be taking rest days during your training. So don't feel pressure to cram it into your schedule and double up on workouts. Pick up where you left off, regain your momentum, and get back on track. Just try to return to your schedule as soon as you're able, since we all know how one or two days off can quickly turn into a week, then two weeks, and then you get to a point where you're feeling totally unmotivated to get back to running.
Also See: What If I'm Off from Running for a Week or More?