The crash happened around 6:30 p.m. Tuesday in the area of South Rio Vista Street and Wagner Avenue, near the Santa Ana River, Anaheim police said.
Police said a driver in a silver Mercedes-Benz was traveling at a high rate of speed and lost control of the car before going off the road and hitting the woman on the greenbelt.
Officers initially responded to a single-car crash, but at some point during their investigation, a man who lives in the area reported his wife hadn't returned home from a walk along the greenbelt.
She was later found dead in nearby brush after a search.
The Mercedes landed in the Santa Ana River. The driver, identified as 33-year-old Raymundo Rangel, and passenger of that car were taken to the hospital with minor injuries.
Authorities say Rangel was taken into custody for gross vehicular manslaughter and felony DUI.