You're right - that's an enormous amount of energy. What you need to do is place (and justify) a limit on power. But let's say there is no such limit.
Your calculations are correct - ~ 4 MJ per day. If she exercises hard, she can easily up that by 50% and not gain weight.
Alynn is a 17-year-old girl. Presumably she was gently raised, and socialized to passivity and avoidance of direct confrontation. Good thing. It is only her mental reservations which keep her from being a frickin' killing machine.
Let's say she carries around a few pieces of scrap iron, or largish pebbles. If she throws one at an attacker, then boosts its velocity, how much energy does it take to do real damage? Well, a 1-oz 12-gauge shotgun slug has a kinetic energy of about 2400 joules. And if she wants to impress, you should be aware that it takes no work at all to keep an object suspended at a constant height. Force, yes - work, no. Work is force times distance; no change in gravitational potential energy means no work done, and at walking speeds kinetic energy is not an issue, any more than it would be if she simply carries the stuff. So she can walk around with a half a dozen rocks suspended over her head, ready to use. And she can use them about 800 times a day (1600 if she doesn't try to reuse them). How big is her village?
The same principle applies to artillery support. If there are any mountains close by, she can move a few boulders from the upper slopes and position them over the village she's living in. This will take awhile, since she can't move them fast, but once they're in place it takes no energy to keep them there, waiting for the command to fall.
Stopping a bullet is harder, but not for reasons of energy. A .45 slug only has 500 joules of kinetic energy, so that's all it takes to stop one, and it takes much less than that to simply divert its path. No, the problem is reacting fast enough to perform her magic on something that's moving too fast to see. Or a bullet that has been fired at her from behind.
In a pinch, of course, she can always do really scary stuff. The human brain weighs about 3 pounds. If she's willing to expend 1/10 of her daily magic ration, she can instantly raise the temperature of a person's brain by about 100 degrees Celsius (just below boiling), cooking it like a hard-boiled egg and killing him instantly. 3 lb of water raised 100 degrees F takes 300 BTUs, or just over 300 kJ. It will, of course take considerably more of her energy to raise the temperature to more than boiling, but the results would be pretty impressive, as the skull would explode. Not pretty, but an effective way to establish "Don't mess with me".
An even more energy-efficient technique would be to drain heat from an attacker. Dropping brain temperature by 20 degrees would cause instant hypothermic coma, and take only 20% the energy that heating would. As a plus (from a certain perspective), the affected individual might well recover, as flow of warm blood from the rest of the body would gradually warm the brain back up.
And if her control is really good, and she's feeling really nasty, the human eye weighs about 7.5 grams, or 15 grams total. Cooking those parts would be no trouble at all (energetically speaking). Total energy to raise one liter of water to boiling is ~ 2600 kJ. For a 15 g pair of eyes, call it 40 kJ to make them explode.