In reply to goosebump:
Hi Goosebump. I don't think you've necessarily made the wrong boot choice. You've just never worn stiff boots before. And I would hope after a few more trips that your feet would get used to it. Your foot is used to flexing in a fairly unconstrained manner with other footwear, and now you've forced it to remain rigid throughout your gait.
I always get sole cramps (extreme) when taking off up hills for the first time in a while in stiff boots. It does ease though after about 30-60mins.
The suggestions of others above all help, particularly leaving the ankle lacing loose, so that your foot can flex more naturally -- until it gets better trained.
And footbeds are almost ubiquitous out there amongst big boot wearers. So that's not a cop-out at all. Mind you, I have extremely flat feet and the arch imposed by some off-the-shelf footbeds exacerbates the problem.
The bottom line is that the effect you describe is fairly common. You want to make double sure, though, that your foot isn't being compressed in strange ways, and is otherwise comfy. No weird rubbing or pressure or anything? If there are no problems there, then they will most likely work for you and you might have to train your feet slowly, and/or try a footbed.
All boot buying is a black art.
Incidentally, what have you ended up with? We tried the womens Monolith, thanks to your advice and were able to compare side by side the womens vs mens on my girl's feet. But the women's toe really does taper down a lot doesn't it! After a few hours of shop walking we stuck with the roomier mens (despite them being too roomy actually) and she has now committed out on the hill.
Cheers
Bruce