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If you head south you will be able to exit at Torrey Pines City Beach, a path leads up to a suburb from which you can easily exit on to N Torrey Pines Road.
Further along south you can walk all the way to La Jolla Shores, a public beach with street access.
If you head north you will run into Torrey Pines State Reserve, from which there is a trail that will lead you back up to the road. Going further north you will eventually hit the entrance to the state reserve. Here the road runs right along the beach.
I can't remember how long it takes to walk, but it is at least a mile either way on sand, so I expect more than 20 minutes.