Why does my lower back hurt when bending slightly over? It also extends to my legs, at times; but that is not as severe as the back pain.

There are many cause. There are many causes, the most likely is a herniated disc. If the pain continues and interrupts your activities, you should consult a spinal specialist.
Conditioning. The pain you described may be related to some early degenerative changes of a disc or facet joints. A good back conditioning program is usually recommended. If the pain worsens, then an evaluation with a specialist to understand the pain is warranted. If the leg pain worsens, it may indicate a progression of the disc to a herniation.
Could be a disc... It could just be a muscle strain, but the radiation into the legs suggests a possible nerve irritation that could be from a herniated disc in your low back. Bending forward can make disc herniations worse, so if that is making your pain worse try and avoid it and get evaluated by your doctor.
See a physician. Severe back pain with radiating symptoms down the legs could be indicative of neuropathic problems of the spine. An MRI may be used to futher diagnose your problem. See a physician as soon as possible.