Remember the rule of threes for survival.
3 minutes without air
3 hours without shelter
3 days without water
3 weeks without food
I am assuming that you are wanting to create a "get home" kit rather than a specific bug out bag since you will likely already be traveling and your supplies and support system will be at home.
With the above in mind you should think about what will be between you and your home. Things such as terrain, average temperatures, availability of water, obstacles, the likelihood of 4 legged or 2 legged predators, medical conditions and how good a shape you are in.
For instance, most of the time I am either at home or work. The terrain is rolling hills with houses, fields and several rather large creeks and one river. My area is relatively warm most of the year other than 3 months or so in the winter and even then, it isn't something that would give me frostbite most of the time. 4 legged predators are basically non-existent in my area (no bears, mountain lions or wolves, just coyotes). There would, however, be quite a few opportunistic bottom feeders if I were traveling by foot after a disaster so the gray man approach is much better than a shoot me first approach. Concealed weapon, small bag, no rifle/plate carrier/big ruck. Currently I do have a medical condition that might cause me problems but it isn't something like diabetes that would be a major setback. I am in fairly decent shape and I walk a lot all the time and I could easily walk 10-15 miles at a stretch. Since my work is 25 miles or so from my house it would take me 2-3 days to get home walking taking appropriate precautions.
So, I have a small bag that has some water purification, some fire starting stuff, a good quality knife, a Glock 19 with a spare magazine and some spare socks, first aid kit and an emergency blanket. I want to be able to move relatively quickly and get home to my wife where I have other supplies. I don't worry about having food (less than 3 weeks to get home) and I don't worry about needing major shelter (temperate area with houses/barns/woods to build shelter/fire), don't need to carry water with fire/purification filtration.
If I were in different circumstances, I would probably have different stuff. For instance if I lived in bear or big cat country, I would choose something other than a G19. If I lived in a major city, I would likely choose to have more ammo or maybe a rifle to shelter in place. If I were a lot farther away from home, I would likely plan on sheltering in place until I could secure transportation.
Because I live in the New Madrid earthquake zone, most of my planning is centered around getting home after major super quake. So I am not expecting to be able to cross bridges or drive home.
