It's your birthday, make your own rules.

My idea of a birthday ride is that on my birthday or the first chance I get after my birthday, I ride my age or more in miles on a bicycle. I try to get at least one extra mile - one to grow on. Rides before my birthday don't count because I'm not that old yet. That would be cheating.

This works for me as I am, so far, able to ride my age in miles without great difficulty. If that ever becomes too much of a challenge due to illness, injury or whatever, I could see making it my age in kilometers, minutes, inches, seconds, whatever.