You said it yourself... you don't spend much time outside.
That IS more than enough to cause Vitamin D deficiency.
I would surmise that the reason you feel fine is because of your biological age and potentially being more physically active than people who are older... therefore, you wouldn't necessarily be prone to problems such as terrible aches, muscle pains, fatigue, etc. (these factors also depend on your environment and how your body reacts to it).
I wasn't going out in the sun a lot in the last 31 years of my life... so I spend most of my time indoors - but I do exercise on a regular basis and try to eat relatively 'right'.
I started supplementing with Vitamin D about 5 months ago (10 000 IU of D3 daily).
I can say that I noticed some changes in my skin... namely it feels smoother, more moist that before (usually I had very dry skin), and my immune system also seems to be operating more efficiently.
Vitamin D deficiency however CAN (and probably WILL) make you more susceptible to various bacterial and viral infections as you grow older and can subsequently result in headaches, aches, etc.
Actually, I would argue you already ARE susceptible more to viral and bacterial infections due to deficiency... but this can be easily remedied.
I wouldn't necessarily go to your doctor and ask for Vitamin D prescription because they could easily give you D2 (which in large doses can be toxic and is far less useful to the body).
What you need is Vitamin D3.
If you want to bring your levels relatively fast... then you need high potency and frequent supplementation.
I would personally advise you to take 70 000 IU of Vitamin D3 twice per week (say every Monday and Friday), or you could supplement with 20 000 IU of D3 daily (its your choice - but I would surmise that supplementing Daily with 20 000 IU would be more efficient since this seems to be an observed maximum saturation level for 1 day).
The amount of time needed for supplementation would be in the range of about 4 to 6 months.
Then you can re-test your vitamin D levels.
I would personally aim at over 85 ng/mL (which would be 8.5x higher than where you are now).
If your new numbers are between 85 ng/mL and 100 ng/mL, then you could reduce supplementation to a 'maintenance dose' of about 40 000 IU of D3 per week (you could split this into 2 doses per week as described above).
Generally however, you could also endeavor to spend more time in the sun... such as 20 mins per day (without sunscreen) between 10am and 3pm (this is for the summer - for other periods in the year, you'd have to spend about 30 to 40 mins in the sun daily... which in today's world is not really practical for most people and why people actually get supplements).
Drink up on some Orange Juice and go get a summer tan, and wah-lah increased Vitamin D numbers.
First off try and relax. It's low D not the end of the world. Get a prescription from your doctor and get your numbers up. I can't tell you why your numbers are low, maybe it is because you don't spend enough time outside. It's good that you are not feeling any negative effects. I have low D and feel crappy alot of the time, I just found out and I am working on getting my numbers up as well. I'm sure you are fine. If you felt good before you knew your numbers were low then nothing should have changed in your thoughts.