As others have already mentioned, it is important to allow your body some rest in order to recover and regain your maximum potential. That said, I myself do not think motivation is as crucial as discipline. The amount of motivation a person may have changes on a day-to-day basis; some days you will find yourself eager to train, while on other days you will struggle to even get yourself out of bed. This is where the discipline kicks in. The truth is you will not always feel motivated to keep going, but it is important for you to move forward anyway. If this is truly something you want to do, then do not let the bad days get inside your head; you have to allow the discipline to take over. Now, as I have mentioned before, rest and recovery are paramount to any workout routine. It is up to you to create this routine as you see fit and always listen to your body as you move forward. While you do have to train hard to achieve your goal, nothing is more important than your well-being. This is why you should adjust your workout as you move forward if you feel the need to do so. An example would be, as others have said, to start slow and increase intensity with time. This will give your body the ability to ease into the hard training ahead, rather than scaring you off. When all is said and done, I can only give you advice based on my own life. Each person is living their own unique life and it is up to you to find out the methods that work for you. Finally, always remember that trying does not guarantee success, but not trying does guarantee failure.