That is actually not true. Pacific Crest Trail is one example. But it is difficult to connect up routes in a number of instances especially in the desert; good route finding skills are required. Additionally, some of these are NOT newbie hikes. If you look into books by Jerry Schad (Afoot and Afield in San Diego County, for instance), you will find oppertunities to connect up hikes/backpacks in local areas. A bit more work because you need to research, but definitely can be done. The easiest 5 to 7 day IS the southern portion of the PCT; the days can be nice and the nights will be cold.

There is a whole set of backpacking adventures to be had just south of the boarder in Baja California as well. Located in wild and rough country, these are also not newbie hikes and now potentially have some new hazards as well. Water finding skills come up this time of year, but one of my favorites is La Tasajera meadow past La Grulla meadow all the way to the apparent ruins of Mission San Pedro Martir. There no guidebooks to this currently in print either (that I know of). There was one by a fellow named John Robinson that you may be able to get used but I wouldn't go there on a lark; you NEED to be good at route finding.