When you’re training and making progress you will hit a point where you plateau. This will happen regardless of the activity you are doing. If you’re improving every workout you will hit a point where you stagnate, where you actually go backwards. This is completely normal and is the reason that many training plans have a week or two thrown in every five or six weeks for rest and light training. Just like rest days are important so are longer rest periods.
When you reach a point where your workouts are draining you more than usual, or it is suddenly a lot more difficult to hit marks you were hitting the day before you have probably hit a plateau (barring any other factor like illness or injury). This is where you need to take a rest and focus on moving past it. A rest does not mean a week of laying on the couch eating junk though, you have to spend a week in active rest.
Work on alternative exercises, try something new and easier or do your normal routine just at a lighter pace or lower weight. Scale it back for a week and then go back to where you left off the next week. This scaling back gives your body time to recover while also keeping you active and in the mental routine of working out. Scaling back should also give you some more free time to focus on rest and stretching. Be sure to spend the week eating healthy as well.
When it comes to eating on your plateau I would say to keep eating the same amounts you were when training, this will help build a slight excess of stored fats and carbohydrates for when you resume training. It will also help with recovering as the body needs food for the energy involved in recovery.
At the end of your plateau week you will find that you have more energy in the gym or on your runs. You will find that getting back where you left off is a lot easier and that you start making gains again. As you reach more elite levels of training the gains will be incremental and a plateau week will not help you much in pushing past the limits of the human body but it will help in reaching your goals.
It is often forgotten by amateur athletes that intelligent rest is one of the most important parts of training. So don’t try pushing yourself when you plateau, rather take a break and let your body recover. You will find that some time off does wonders for your performance.