I recently got an email from a beginner runner who's in her first week of the "3 Weeks to 30 Minutes of Running" program. She missed a day of training and wanted to know if she should start over or pick up the training where she left off.
It's easy to get discouraged if you get busy or sick and you miss a day -- or several days -- of training. Don't beat yourself up for missing a run - life happens. You're not going to suddenly lose fitness because you missed a workout! So don't feel pressure to cram it into your schedule and double up on workouts. Pick up where you left off, re-gain 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.