Your credits may or may not transfer … you could discuss with U of Ariz if you are thinking of this path and see if they would allow you to complete your PhD there with ample credit for the 10 classes you take for your masters.
That said, I actually took 4 class in the full-time program at JHU and then finished my masters through the Masters of Engineering program at satellite campuses at night. The full-time program classes were much more academic and rigorous, some of the others may have sufficed for a PhD program, like computational methods and dynamics. The part-time program was taught at a lower level which was appropriate for people who were working 40 hours a week, commuting to work and school, and probably raising families.
I think you would need to really push yourself in the on-line program and take rigorous classes that are required for your PhD.
That is a long road, by the way, count on 10 classes taking 2.5 years, then a PhD program maybe another 4… but you are earning a living at least for part of the time …