Regulable Metabolism (Thermoregulation)
You never know when you will find some food. Warm-blood animals has a higher metabolic rate, they are faster, stroger and more actives, but they consume more calories. Instead, a cold-blood animal has a very low metabolic rate, consuming much less calories, but at the same time being too slow and inactives, they won't be able to fight.
But, you could have an advances metabolism able to regulate itself. During low-food or idle time it decreases the metabolism (e.g: reduce inner heat from 42ºC to 10Cº saving 30% energy, NASA said that a reduction of 10ºF in human temperature could reduce human metabolism around 50% - 70%), and during high-food or active (hunting, fighting) time it increase the metabolism.
That is a more extreme version of bradymetabolism, I am talking about a creature able to switch from:
- From Endothermy (generate it own heat) to Ectothermy (get heat from enviroment).
- Drawback: Ectotherms creatures must get heat from sun light, that means that in an Nuclear Winter it may be difficult to use, or at least they may need other heat source, like a geiser or something like that.
- From Homeothermy (regulate it own temperature) to Poikilothermy (don't regulate, survive at several different temperatures). Also know as Heterothermy (homeothermy at day and poikilothermy at night).
- Poikilotherm creatures has several "sets" of enzymes in they body, each set especialized to an especific range of temperatures (instead of just one). That means that their bodies are less efficient because they have "redundancy" on enzymes, each one isn't especialized in an X temperature, they are especialized to a range X - Y of temperatures.
- This evolution is really unlikely to evolve from humans without some handwavium, because this need a complete rework from all our systems (you have to reconfigure all the production of enzymes).
- Addition: Like a bonus, your humans could have some rete mirabile in their bodies. That is a complex of blood vessels with a lot of different function, but in this case they could be used with countercurrent heat exchange in order to focus the body heat to the chest (the most important part) instead of the whole body. (It transfer the heat from arteries from the heart, to the cold veins from the limbs to the heart).
During time of food shortage your "creatures" (if we add all this modifications they will not be humans anymore) will need a good way to save calories.
This can be done with Torpor or Dormancy (Also know as Hibernation on winter, Aestivation on summer/desert) or Brumation on reptile hibernation).
Normally, people think about hibernation (Torpor) as an ability to reduce the metabolism during days or weeks, maybe more, but not less. That is the wrong idea, torpor can be used even for just a few hours, like while you are sleeping!
With torpor they could induce an slow metabolic rate to consume less calories, and if you know about the shortage even before it, you could increase the amount of fat on your body, and then reduce your metabolic rate in order to conserve that fat for more time.
- Drawback: It's also quite difficult to explain, but not so much.
- Also, an extreme version of this could be Cryptobiosis, instead of reduce metabolist rate, it stop metabolism. Literally, the creature stop (like put pause on a movie), and when the enviroment is fine (e.g: normal temperature, water or oxygen conditions) it reactivate itself by some chemical reaction. This could also be used to survive really, but really extreme conditions (vaccum, freeze, boil), or at least tardigrades can.
This is basically the opossite to the ideas above. Here, you never know when you will be attacked, so you have a high metabolic rate even while sleeping (note that this rate is still many times lower than awake, but no so much as in normal animals). By that way, even sleeping, you can wake up quickly and start running or doing something almost instantly, at the expense of a lot of calories.
Split Brain Sleep Times
Like dolphin, your creatures must be able to split the day in three phases: full mode, during this time the brain work normally at full power, half mode, during this time only the half of the brain works, the other half is sleppend, and another half mode, where the sleeped part is awake, and the awake is now sleeping. With this ability, your creatures can always be on alert.
Sleeping 8 consecutive hours during night can be dangerous... someone could atack you. Your creatures could split their sleeping time to several smaller sleeping hours.
In a nuclear fallout scenario it can be a problem the radiation.
- Having a denser skin (fat?) could be used to absorb incoming radiation and focus on a less vital place (skin instead of inner organs). Their skin could be adapted to suppress damaged cells quickly.
- This will be extremelly useful to resist alpha... the less dangerous radiation... if it isn't inyected, ingested, or inhaled (otherwise it's the one of the worst).
- Extreme advanced DNA repair: For example, tardigrades -an ugly microorganism but extremelly difficult to kill**- is able to survive 1.000 times more radiation than normal animals. A human can survive around 5 - 10 Gy radiation, while them can survive around 5.000 - 6.200 Gy. One of the reasons to be able to do that is their advanced DNA repair system that their cells have.
The ability to see in darkness has obvious advantages, and this can be adquire having tapetum lucidum on the eyes (increase light avariable on photoreceptors, cat use it to get 44% more light). Another advantage of the tapetum lucidum is the eyeshine of some animals. Furthermore, having ultraviolet sensitivity can improve the night vision, foraging, prey identification, and frugivory (fruits and berries reflect UV light
UV Protection and Protective Layer (harder skin)
If the world has a very damaged ozone layer it could be useful been able to protect from UV rays. The skin of that creatures could have Beta-Carboline in their exoesquelet layer of keratin like Scorpions, in this articule (Warning: Spanish) it speak of some hypothesis why they have that sustance in order to be fluorecent, they think it may be an UV protection of some daylight scorpion ancestors (because it absorbs UV ray and produces cyan light).
- Drawback: This reflective layer is like a kind of exoesquelet, it can be useful for defensive purpouses, but it will reduce mobility.
Efficient Digestive System
As we learned the fire and how to cook meat, our digestive system has specialized in order to eat cooked meat and their size as been reduced because cooked meat is more efficient kcal/kg so we don't need a so long intestine (that consume also innecesary calories which now are used to feed the bigger brain)  (I can't find that article). But, when the food is low, you need the most efficient digestive system.
Advanced Inmunity System
In an apocalyptic world medicine is something very rare, and a simple wound could become in an infection -> amputation of limb -> futher infection -> death. If our imnunity system were stronger it would reduce the chance of infection, also it will cure diseases like cold faster, increasing chances of survival.
I will quote something about the Thymus, who produce immunity cells:
The thymus is largest and most active during the neonatal and pre-adolescent periods. By the early teens, the thymus begins to atrophy and thymic stroma is mostly replaced by adipose (fat) tissue. Nevertheless, residual T lymphopoiesis continues throughout adult life.
But... what if that T lymphopoiesis (immunity cells) die from a disease like HIV? If they Thymus never stop producing cells (or at least it stop but it's able to re-activate later) that creatures would be able to defend themselves from disease that immunodeficiency.