There is a wonderful series of paths along the water, running from the Aquarium in Long Beach for 7+ miles south to the Peninsula in Belmont Shore. The path is paved most of the way, and there are always lots of runners and other outdoor enthusiasts enjoying the great water views and other scenic attractions along the way.
You can do any portion of this path you like. Starting at the northern end at Shoreline Aquatic Park near the Aquarium off Queens Way, enjoy a loop out to the Lighthouse and then a 1-mile loop out to the Long Beach Marina, where the Los Angeles River meets the ocean. Then enjoy a nice 0.5 mile stretch along palm-lined Marina Green, reaching the beach at Long Beach/Junipero Beach. The paved Shoreline Way path runs literally on the (also runnable) beach for 2 miles (alternative: path along palm-lined Bluff Park, above the beach). Then, enjoy jaunt out to Belmont Pier (0.5 miles out and back). Continue along Shoreline Way, passing the outdoor Belmont Pool, volleyball courts, and Rosie’s Dog Beach. The last section is ~1 mile out the Alamitos Peninsula, where you can do a loop of both sides of the marina, enjoying nice views of the water and fancy boats.
Add-Ons/Alternatives: Near Marina Green, cross Shoreline Dr. to do a 1-mile loop of the pleasant Rainbow Lagoon Park. At the end of Junipero Beach, a nice path runs along Bluff Park for 1 mile. At the the Alamitos Peninsula, you can take Bayshore Ave. to enjoy numerous pleasant roads and paths around the Marina.