Several years ago, my vitamin d serum level came in around 5. I don't remember having a lot of symptoms of deficiency aside from fatigue and slower times (although I'm a pretty slow runner to begin with). At the lab where my tests were done, the optimal range for serum vitamin d levels is 30-80. Now I take 50,000iu weekly in a gelcap and my vitamin d is at 50.
Many factors are involved in our synthesis of vitamin d including latitude, skin pigmentation, the angle of the sun, etc. So even if you spent a lot of time outdoors, you may still have a deficiency and should get tested.
Here's an interesting video with Dr. Michael Holick which explains why we need vitamin d hormone and how we get it. It's 58 minutes long but worthwhile.