Missing runs during training can be quite a drag. Whether it is due to illness, spending too much time at the office, caring for a sick child, or anything else, it prevents you from enjoying the run and, if you let it, can put a damper on your motivation.
If you miss a week of runs during a training period, do not try to make up for lost time when you start back. Just move on as if you ran that week and pick up with the current schedule. However, start back at a slower pace to allow your body to adjust, especially if your time off was due to illness.
From a mental perspective, view the time off as a positive. Do not let yourself get discouraged simply because you missed a few runs. No matter what their level of dedication, many runners miss runs because things can arise that we simply cannot control. We can, however, control our mental attitude. Choose to see the time off as an opportunity to rest, perhaps something that may have helped you avoid an injury, or just think of how great it will feel to achieve your goal in spite of the unexpected obstacles that came your way during training.
You can achieve your running goals even if missing a few runs. Never let something you cannot control bring you down.