Re: Any singers here?
I used to sing Classically. I'm not going to give you specific advice on technique (because I am most certainly, not qualified) other than to say never, ever strain your voice. If you begin to feel that material is going to 'tire' your voice and you are singing it with force try to get a teacher to assess your technique and vocal range. With training you can improve your range (both up AND down) but simply straining will not help you.
Curiously my singing voice has developed a much higher top-end. It broke when I was eleven and I'm twenty-three now. If you'd had told me four years ago that I could hit a top tenor 'E' you would have me laughing. With practice and warmup I can now. Range is certainly not the be-all, however. Control and listening are more important and that's where I can sometimes struggle.