vocal strain usually stems from improper vocal technique. You need to look at the production of the sound you are creating, and the tension that you carry in your vocal chords while you sing. I sing 4-5 hours/day 5 days/week in my job and have so far avoided any vocal problems. You may find it beneficial to seek out a GOOD voice teacher who will help you understand the mechanics of singing. This involves tongue placement, air flow and a myrid of other things. BE SURE TO FIND A TEACHER WHO UNDERSTANDS THESE THINGS, AND DOES NOT JUST TEACH SCALES AND MUSIC. The wrong teacher can destroy your voice. (I know people who have had this problem, even from university teachers. I'd offer my wife's services, but we live in Vancouver, BC. Good luck, and take care of your voice, it's the only one you've got. You can't go and buy a new one when this one breaks.

Paul Evenden