A 100 mile unsupported (no re-supply stops) hike is very very doable if you pack your food well. You also have to plan out your water re-supply points very carefully in the front ranges of California.
100 miles is 15 miles/day for a week or 10 miles/day for 10 days. I personally would plan at least one "zero day" (rest day where you do zero miles) in there.
Some menu classics:
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Quaker Instant oatmeal. It can get boring after a week, but it's DARN good on a cold morning, and it's light and packs well. The wrappers can be burned (where safe) or packed out. I like bringing along a packet of raisins which I mix in. A little extra weight, but TASTY. Mmm!
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Ramen. Very light and very tasty, especially if combined with dried veggies (which re-hydrate just like the noodles). Get Sapporo Ichiban brand not the inferior Top Ramen brand. Sapporo Ichiban is available at most Asian markets and at a lot of non-Asian markets.
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Knorrs brand rice or noodle mixes are a nice, tasty (and cheaper!) alternative to freeze dried backpacking specific food. I supplement with nuts and jerky since the Knorrs packets don't have much protein. If you really want a nice meal, mix in a packet of chicken. It's not light weight since the chicken is not dehydrated, but it's a delish combo.
Let us know how it goes!
HJ