Here is an equation that dawned on me today while running. It is quite simple.
a) Speed at which you can run.
b) Distance that you can cover.
c) Running without any pain or injuries.
You can only have two of the above.
Obviously c) has to be selected - cannot imagine a life with pain - unless you are Zahir Khan or one of the sissy Indian cricketers who always feign pain in order to skip playing and go act in more Ads.
That leaves us to pick either a) or b). So if you have only 30 minutes to run - run as fast as possible for a short distance. If you have lot of time on hand - run as far as possible - but slowly with enough breaks.
At times these constraints are a real PITA - but that's life - we should just try to make the most of it.