Sociology aside, this is a realistic scenario
(or at least as realistic as Robin Hood)
In this answer, I'm setting aside the fact that Poland in the 1400s would not be a conducive sociological environment for this type of thing, since your character's backstory presupposes that she's grown up in a very non-traditional manner.
First, I'll suggest a setting and mode of operation for these archers, and then expand on why I believe it's plausible given the constraints of the situation.
Winter. A forest in the foothills of a frozen mountain range, either the Sudetes or the Carpathian. The daily temperature rarely strays higher than ten degrees below freezing. The forest is thick, and the snowcover is thick enough to bury a man to the waist in the deeper drifts.
(This isn't strictly necessary, but it will make for a dramatic setting and adds an element of danger from nature itself, so I am going to go with it. Much of what is below applies anyways.)
Like you suggested, your archers are going to be using ambush tactics. Your protagonist is the daughter of a forester, and is fighting enemy troops in woods she's explored since she was a little girl. Her main tactics for combat are going to be ambush attacks on sentries or opportunistic attacks on lone messengers and small groups of enemies, firing arrows from within tree canopies, and fleeing on snowshoes, skis, or through the trees.
They will have light bows. 50lbs? Nah. Their bows (your protagonist aside) should be 30lbs, max. That's the sweet spot draw weight for small-framed women. It's enough to hunt deer, (though heavier draw weights are preferred) and will easily kill or maim humans.
A lighter bow cuts the training time by a lot, and while a 50lb bow may be stronger, it's not necessary, either for range, or kill potential. For distance, if there are 30m of open, unobstructed space between them and their target, they are too exposed to be safe. Most of their kills should happen from an elevated position, at a range of 10-20m, in dense parts of the forest. They should not be trying to punch through armor, they should be aiming at any unarmored area. An arrow in the eye or throat will kill you just fine...
But an arrow in the thigh is great too. They don't actually want to kill their enemies, necessarily. A dead enemy is good, of course... but a wounded enemy who takes four weeks to die of a gangrenous infection in his leg from an arrowhead smeared in feces is better.
Most importantly, combat is only a fraction of what they'll do, since the best way to survive a fight is to not be in one.
They'll dig punji pits and troupe de loups on roads the enemy uses, hidden under the cover of snow. They'll sabotage trail markings and steal supplies. They'll goad hounds to howl all night, so garrisons or camped troops get no rest. They'll sabotage bridges across frozen streams, so the timbers snap under the weight of twenty armored soldiers, crashing through the ice, soaking them, and leaving them to freeze to death. They'll find out enemy positions and encampments, and pass that information (by proxies, likely) to any formal military opposition in the area. They'll warn villages of approaching soldiers, and help in denying their enemy the ability to feed off the land. Also, at night, they'll steal, kill, and, if possible, eat horses. (but don't put that in the book. People don't like animal death).
The land itself will be trying to kill these invaders. The girls are just helping out.
For the tactics that your guerrillas are using, gender really isn't going to be a limitation. The main points of comparison are going to be:
- Endurance. The ability to keep going day after day under harsh conditions. Women match (or sometimes exceed) men at this. Not in marathons, but in ultra-marathons.
- Climbing. Your girls are going to be climbing trees, rocky ridges, and walls. In climbing, women are not at a significant disadvantage with men. I can't find any stats on tree climbing, but I imagine that would apply even more here, since they are significantly lighter.
- Riding. Women are comparable horse-riders to men. Hell, the Riders of Rohan were mostly women in New Zealand. (Though I recognize that modern hobby equestrianism isn't equivalent)
- Shooting. I'm specifying shooting before archery because the most important thing here is accuracy, not poundage. In shooting at the short ranges that forest combat will have here, (10m) women beat men.
- Archery. Men regularly beat women in archery competitions by a significant enough margin that they don't compete against each other (mostly)... but that margin is still very small, often less than 5% in points (and certain world records are equal)
- Cold endurance. There is some (limited, disputed) evidence that in long-term cold endurance, women beat men. (Though in short term and in frostbite resistance, not so much). Whether this is true in the mean, in this case, is less relevant that whether it's true in extreme cases. Chloe McCardel has swum across the English channel 44 times (the most of any human in history). The record for most (though not fastest) there-and-back-again swims across the English channel is also held by a woman, Cindy Nicholas, at 5. At least when it comes to dedicated acts of madness combined with cold temperatures, women certainly hold their own. A winter-long murder spree in a frozen forest certainly qualifies.
For our protagonist, we take as given that she is a proficient archer. The question is, how quickly can her recruits pick it up?
The fact that we are setting our targets at 10-20m is huge. With a decent bow, (maybe she's also a very good bowyer?) I'm fairly confident that a good archer with a deep understanding of the weapon could teach a teenager to consistently put >90% of their arrows into a medium sized target at 10m in two to three hours. That was my experience when I learned, and some googling around shows that it's not an uncommon number, and some are optimistic it could be done in an hour. (Maybe I was an especially poor student).
A month of practice should be more than enough for them to get >50% bullseye hits at that range, and if their target is a 10cm wide unprotected face... here is an unfortunate mercenary's face plastered on a 50cm target.
That center bullseye is 5cm across —just double the width of your eyeball.
They are certainly not going to be fat, because that simply wasn't a thing people other than nobles really got to be. Especially in winter, and especially in Eastern Europe, and especially during wartime.
They are going to be thin. They are going to be filthy. Their skin will be rough, their faces windburned, their eyes hollow. Some will be missing fingers and toes.
Something a lot like this. Complete with the skeleton trophies.
Three Russian women sit behind a table piled with human remains, taken in the Volga region during the 1921-1922 famine.
Taken from the Russian government archives, the Peter Struve collection, box 25.