Death can occur: 3 minutes without air, 3 hours without proper shelter, 3 days without water, 3 weeks without food. Of course, these numbers can go in either direction, but they are a start.
Look at things from an easy start point......the Five C's of Survival; Cover, Cordage, Container, Cutting, and Combustion. Cover; that;s the casualty blanket, as well as the anorak. Cordage; that's the 50ft of type 1 paracord (dummy cord). Container; that's the 500ml water bottle (probably Klean Canteen Stainless single wall to be able to boil....could be a Nalgene with stainless cup), plus the squeeze bag and filter. Cutting; well I at least carry two, if not 3 knives/tools. Combustion; bic lighters, plus matches/ferro rod.
Dave Cantebury takes it up to 10 C's, with the addition of Candlelight, Cotton Bandana, Compass, Cargo tape, and Canvas needle. Candlelight; that's covered with the EDC surefire. Cotton bandana; that's easy enough to add. Compass; I've talked about adding that, plus a good topo map. Cargo tape; (aka duct tape) I've got a miniature roll of gorilla tape to throw in. Canvas needle; I'm not looking at being out there that long, probably the one thing I would leave out.
3 hours without proper shelter. First is to stay DRY, then to stay Warm. You can go into hypothermia in as high of temperatures as in the 50s, by being wet and having a wind. So, you get out of the wind and rain, and either bundle up in a blanket to keep warm, build a fire to keep warm, or BOTH. Cutting, Cordage, Cover and Combustion all work for this one thing, to maintain core body temperature. I could easily add in 2 or 3 of the mylar space blankets to aid in all of this.
3 days without water. This all depends on a lot of things. In the scenario I'm talking about, I'm MOVING, through the woods, when it's hot and humid. To ME, water becomes much more important. Then, there's the difference between being "alive" and actually functioning. Of course the container goes to this, but so does combustion (to boil/purify), cover (rain water catchment), and cordage (more rain water catchment....drip lines).
So, as long as you can stay dry, relatively warm, and hydrated; now you can live for 3 weeks or more (with my added padding, I'm saying 5 weeks min.).
The first aid kit is of course to treat trauma, but more importantly to prevent little things from becoming BIG things (scratches/cuts from getting infected, etc.). The anorak is to allow me to continue to move while trying to stay dry. it's also big enough that it covers the pistol belt/pistol, in the event of rain.
SO, this is a BASIC kit, to walk around in my local area, for at least 2 if not 3 seasons (could be pushed into winter, if it's not too harsh). Anything else COULD be layered on top of this, from 24/72 hour assault packs, to full blown rucks. With an assault pack, I would probably attach a bigger "bush knife", such as the Esee 5 or 6 and probably throw in a larger folding saw, like a Silky Professional 5". I would probably also throw in some easy "food", either cold drink mix (Gatorade or the like) or hot drink mix (coffee/cocoa), soups (lipton), trail mix, hard candy and the like. Nothing that would take a LOT of prep, but also nothing too HEAVY, like MREs.
Also, with the LBE I'm describing, it would be VERY easy to throw a plate carrier over the top. All the shoulder straps on the LBE is doing, is helping to hold up the belt; they're NOT padded in any way, and should pretty much disappear under the PC.