Aside from the usual, "enter a triathalon/marathon/etc", how about trying to improve a different component of fitness, like flexibility or cardio capacity?
My goal for age 40 was to be able to do a split and, after adding yoga, was able to reach it. My goal for 41 was to be able to do 5 big girl pushups (I'm HORRIBLE at pushups!) and I reached that goal too. For 42, I'd like to be able to do a "teaser" because my core could use some strengthening. I try to focus on fitness goals as opposed to aesthetic or competetive goals.
Or, if you have time, you could take up a new sport/hobby and become relatively proficient at it. I like setting goals! It's so motivating when you reach one.