I've had to do classes and amass credits as part of my PhD. The accepted wisdom among my cohort was do enough to get a decent grade but no more unless the class/module is really important to your PhD. It's difficult if you are used to pulling in the high grades but it's important to balance what's required of the classes with your own work. They do get you writing from the start and I have made it my business to copy 'n paste at least one paragraph from different assignments into my final thesis, so as I don't feel that they were a total waste. My supvervisor supports structured PhD programmes and yet advised me to 'just do enough' so it sounds as if you're doing fine. Some of the classes were good, some were not but my goodness I'm glad they're over!
A 'B' is good :-)