If you need more exp per level a person could just min/max exp and the game would end up not being much harder.
An average person is not going to become the clone of Bruce Lee, Macgyver and Chuck Norris in three months like your average 7D2D character. So, not being as super powered, is the only fun way I can think of to make the game harder.
You'd be surprised at how fast people can advance in a skillset when they don't have the distraction of job, spouse, offspring, social and or professional obligations.
Consider that a self defense class lasts six weeks to three months. Typically the class meets for one to three hours once or twice a week. Well call it 4 hours a week. That's 24 hours, for one that's only 6 weeks long. For 4 months (A semester) it's 48 hours. 48 hours of training to make someone decently competent at defending themselves.
This includes technique, discussion and theory, as well as drills and sparring. Maybe some realistic simulation training for the really good ones. Put you in a realistic space, and go about as hard as they can short of killing you, to make your adrenaline spike, and help you ride it out, and maintain your head/senses. No panic, keep from getting tunnel vision.
That's weeks, if not months of classes.
But YOU are living in a survival situation. It's constantly fight or flight. Constantly assessing the space around you. There are no rules. Just kill or be killed. There is no substitute for real experience. And you're in it twenty four hours a day, seven days a week, three hundred and sixty-five days a year. By day two you have killed plenty of 'people'. Lets say you, in theory, sleep for 8 hours each day (not likely) that's 16 hours a day where you're on the alert.
And if fighting is crucial to survival, its going to be on your mind ALL the time. You're in a heightened state all the time. You're
ready all the time. And you know how to not get hit pretty quick. You know when and how to take a hit. You know when and how to hit back and where. And you get better and better at it pretty fast.
Let's say you spend only 8 hours per day fighting. In 3 days you have caught up to the chucklehead in a basic self defense class, but the training is far more intense. In 6 days, you've caught up to someone in an intensive class, and the training is still more intense.
Now lets say we expand this to include martial artists. Same numbers 4 hours per week in class. Lets say they train two more hours a week outside of that. So 6 hours. And lets say it takes 7 years to gain black belt (one year per rank,.. averages). Thats 2184 hours of training. Thats 273 survival days, but the training is way more intense. A little over 136 days if you spend 16 hours a day at it. 109 if you spend every waking moment, with only four hours of sleep. I know of some schools that you can attain such a rank in only a year or two, which is 312 or 624 hours. Which you can achieve in a few weeks in game.
You certainly could be Chuck Norris or Bruce Lee. And that doesn't take into account time spent with firearms, as well as scrounging and/or building.
I don't think that advancement is particularly unfair or unachievable. In a world like this you move well, or die. Sub par, or sub standard gets you killed. You do your best or you die. That means you progress skills as fast as you can. Every mistake
might kill you.