Based on some theory, some experience, and some modeling, I've come up with a simple formula to scale running results and allow a fair comparison among people of diverse ages, sizes, and shapes:
|age + body mass index - 50 + 10 (if you're female)|
The result is a percentage adjustment. Take that much off your time in a race, and you've got your scaled result. For example, with a BMI of 24 and an age of 51, my current handicap is 24+51-50 = 25%. So when I run a 10 minute mile I get to subtract 25% from my finishing time, or equivalently add 25% to my speed. It's like doing a 7:30 mile when I was half this age --- which makes sense and feels about right.
The logic, in brief:
One certainly could design a much more complex equation, and likewise could quibble over various of the terms in the formula --- but age+BMI-50+10 (if female) is probably about as accurate as the uncertainties in the input factors allow, and it has the singular advantage of being trivial enough to compute, just barely, during the final stages of exhaustion during a long run.