"This multi-use trail runs north-south, stretching from Azusa to Seal Beach. Though the trail travels through an urban environment, adjacent parks and natural features help diversify the landscape. The San Gabriel Mountains in the distance provide a scenic background for the northern portion of the trail, whereas the ocean serves as a destination point to the south. The trail is a popular bicycle route."
The trail is 35+ miles. Since the northern terminus is about 670 feet higher than the southern, it's likely a southbound run will be fastest. A detailed "Quick Guide & Map" is here: https://trails.lacounty.gov/Files/QuickGuide/SanGabrielRiver.pdf