I always consider running the all-time test of fitness, so I try to do a timed 5K once a month to test my progress. I think that if you consider yourself "in shape," even if you don't run all the time, you should be able to easily complete a 5K. Testing yourself once a month also gives you motivation to beat your last time. All you need to do is map an easy course that is close to 3 miles and repeat the same course every month. Running outside, or at least around an indoor track, is way more exciting than a treadmill. Be sure you have a stopwatch to time yourself and stop the clock if you run into an intersection.
My monthly 5K has really helped me see what shape I'm in (much better than weighing myself). November's time was 26:53. I couldn't run in December due to a foot injury. Then I decided to do January's race this week since it was so nice outside. My time was 27:33, which is ok but could be much better! Since my foot doesn't hurt anymore I'm looking forward to running more and getting a killer time in February!