sprained quadricep

Hi
I have sprained my quadricep muscles and now can't do my 10k at the weekend......... awww...... I was looking forward to it..... looks unlikely i will recover quick enough to do my half marathon in November as well - but that was always a bit ambitious and also there is a good flat March half that I have my eye on...
better for timing...
I don't know how i sprained my quads except that i went too fast too far too soon - i did 12.24 min miles for 7 miles and this is FAST for me, I beat my 5 and 6 and 7 mile times, infact i smashed them...
So I guess because i didn't pace myself or pull out after 5 miles? god - I have a feeling that I will have to go so carefully and slowly with this running thing - 7 miles is my max distance, and now sprained quads -
:-(
my doc said i was -1.1 T score where -1 to - 2.5 is Osteopenic and -2.5 and below is Osteoporosis - for each age group - so it's a borderline score which is good!!