a balance between maintaining a sane level of comfort and challenging yourself
Yes, it's definitely important to keep the balance!
But... I think it is also possible to grow staying in "the old", the known, the dull, in the too well known routine. As WE change from moment to moment, we can discover new things walking the same path over and over... and... over again.
In my opinion, the breakthrough may happen when the routine gets so mundane, so familiar that we actually cannot stand it anymore and... we brak, or rather break thru and unexpected happens...
I think that for example 200WaD environment provides this kind of experience for some of us, because writing every bloody day for months might really be, though smallish, but quite challenging routine.
Some start to experiment, like the absolute slump king, @jasonleow . Some play. Some try using words as more than just carriers of meaning. Some, struggling to maintain the streak, create parallel universes, for others to visit.
I believe that, just as confusion and out of the comfort zone journeys, the dull and repetitive can push us beyond ours, perceived, limits.
As there are NO LIMITS, they exist only in our minds...