I don't want to do it unless I can run it in 3:30 or less.
Why this magic number?
That is the time which would qualify me to run in the Boston Marathon. It seems to me that I should be able to (at least) meet the minimum requirments for such a race, to have any business running 26.2 miles. The problem is that I have only raced at the 10K distance the last few years, so how do I estimate what I could do at the much longer distance? (A marathon is more than four times the length of a 10K) Google grants wishes: I found very quickly (first search attempt) a website which has such a calculator.
It converts my lateste 44:41 10K into 3:25:20 for 26.2 miles.
Yay! It looks as if I have a reasonable chance of meeting my goal. Now I just need to pick-out a marathon held in New England this Fall.