Playing a song or singing a song is a bit different than technical exercises which are much difficult to do long hours, so before practising a student must fix his mind to think that he will give his fixed time in exercises and then later on the song. Songs can be entertaining but technical exercises are improvement boosters and it can help to develop so much, be it guitar or piano or singing. Lastly, whenever we repeat lessons for long we have to meditate on it with the full focus instead of letting the mind wander away from the real path of practice.