Hello and welcome to COL
This may sound like an odd question, but does your girl know how to switch off and relax? It doesn't come naturally to many Cockers, especially pups that have lots of stimulation when at home or on walks (busy household, ball chasing on walks as two examples). Long walks may not be a good thing for her at the moment, for two reasons. One is that her growth plates will still be in the process of closing and her bones won't have the density and strength that comes with maturity, so long walks can increase the risk of joint and bone problems from a young age. The second reason is that she may appear to need even more exercise as she becomes used to what she's already doing.
Have you tried giving her a long lasting chew or a kong after a walk? Chewing releases endorphins and helps most dogs to relax, so it may be one way of helping her to chill. Another is to make the last part of the walk a quiet time so she has some time to start winding down before arriving home.
I agree with sodpot2000 as well, that things like scent work, or activities that challenge her brain, can work better than loads of walking, as long as you help her to wind down at the end of the activity.