Hillcrest is a pretty great area for bike riding. Lots of shops, restaurants, and "bikeable" destinations. That being said, it is fairly urban, and you would be sharing with cars, pedestrians, buses, etc. Caution and preparation is key.
Probably my favorite place to bike around in San Diego is Pacific Beach, which provides plenty of boardwalk space and an unbeatable ocean view to boot. It's also got plenty of bikeable spots -- restaurants and shops galore.
If you are more into nature trails, I can't recommend San Marcos highly enough -- miles and miles of untouched trails and ocean views. Fresh air, cool breezes, can't be beat!
The areas is relatively flat and is a popular village here in San Diego. As the others have shared however, it is very crowded, with loads of cars, buses, and pedestrians. I can't vouch for safety.
I do see a lot of people bicycling at Mission Bay's Park, Balboa Park, Penasquitos Canyon Trails, and other 'off-road' recreation facilities.
This is the contact information for the Balboa Park storefront station and they could answer your question. 1549 El Prado, San Diego, CA 92101 (619) 685-8206
If you need help finding a home I would be glad to help you.
Local Help with Your Real Estate Needs
The San Diego Property Shop
CA DRE # 00648687
-As a former employee of a bicycle shop and nearby resident and bike rider of the area, remember bicycle safety starts with a Helmet, both Front and Back lights for your bike(the Law at night), possibly reflective gear, along with reflective strips on your bike. Make sure your bike is in good riding condition and yes there are potholes in our fine city, I suggest a thicker road tire with run flat or puncture resistant. Popping a flat can endanger city riding. All that said, check the local police department for bicycle accidents vs. cars, it happens everywhere. A lot of bike riders in NorthPark also. Hope that helped?