Mark Allen had an article I recall that addressed this. Here is the link
Craig Alexander (AUS) earned a narrow victory over Daniel Bretscher while Emma-Kate Lidbury (GBR) dominated the sixth running of The Oread Ironman 70.3 Kansas,
www.duathlon.com
Here is the excerpt from the article
Here is the formula:
1. Take 180
2. Subtract your age
3. Take this number and correct it by the following:
-If you do not workout, subtract another 5 beats.
-If you workout only 1-2 days a week, only subtract 2 or 3 beats.
-If you workout 3-4 times a week keep the number where it is.
-If you workout 5-6 times a week keep the number where it is.
-If you workout 7 or more times a week and have done so for over a year, add 5 beats to the number.
-If you are over about 55 years old or younger than about 25 years old, add another 5 beats to whatever number you now have.
-If you are about 60 years old or older OR if you are about 20 years old or younger, add an additional 5 beats to the corrected number you now have.