I haven't worn any Underworks one longer than a couple hours, and that was the Tri-Top, not the Double Front. But I've had bras that started hurting faster, and this is considered "normal" in the world of women, for some bizarre reason. The T-Kingdom binders I've been wearing are more comfortable than bras, period. They don't bind as well as the Underworks, though.
If it's your ribcage and not where it's binding, then that part might stretch as you wear it more, with putting it on and taking it off? I think some of the other guys have mentioned doubling it up, too, so you've got the bottom part binding as well, but I guess that depends on what you're wearing, you could try that if it starts to get uncomfortable?
May as well stand up straight as soon as you put your binder on - once it's on, you're not going to poke anyone's eye out with anything, so walk tall!