40L is not to small for an 8-10 day trip. Or even a thru-hike with a resupply every 8-10 days. I did a 14 day trip along the NPT with a 2200ci(35L) Murmur with a 32F outfit. A similar 20F outfit would have taken about 2500ci or about 40L. But you would have carried 4 days less supplies (mostly food and fuel.)
Typically, my pack weighs about 11oz. This is a Gossamer Gear Murmur. A similar Gorilla fared just as well, but really, I didn't need the extra space, I brought luxury items: a saw, water shoes, extra fuel for frying fish, some fishing gear, etc...
Depending on your gear, you can easily go lighter or smaller. Small light, small stove, bulk packaging (oatmeal in one bag, rice, noddles or pasta for example, maybe a salami or high fat substitute,) crushed Fritos or potatoe chips, chocolate bars, small pad, high FP bag (compressed,) heavy weight long johns, sleeping socks, shorts, shirt(both for doing laundry,) rain gear, etc...
Edited to add:
Yeh, I had forgotten that. A good jacket for really cold evenings, nights and mornings when you are not moving, yet. Mine is good to about 32F in the morning when coupled with a long sleved shirt (fleece or long johns.)