Sounds like you're like me... don't know where to draw the line between pushing your self enough to improve and pushing yourself too hard.
Most people will say "listen to your body, it will tell you how far or long you can go"... But that advice doesn't work for me because I'm tone deaf to my body. So I need to use rules and guidelines to make sure I'm not pushing too hard to far to fast.
That being said, I personally prefer to go for time. Of course I like to run a round distance, but if I have to cut things short, then I usually try to make time be my guide. If I can say "I ran 45mins" then I feel like it matters less how far I ran and more that I did the time.
One time I was running intervals with Lisa and Michelle and when we were done I said "Hey we've only done about 39 minutes of work... I need more time!" They thought I was nuts... but for me, rounding it out with another 21 minutes of light jogging just seemed to make the workout more complete.
Just, please don't push yourself too hard. Remember, I've gotten two stress fractures from pushing too hard.