I was over an F cup-wearing size 9 jeans. So I totally hear ya. It was damn near impossible to run in an emergency! Plus a herniated disk in my neck and a bulging disk in my neck ( can ya guess what they think caused that?).
I was afraid that walking wasn't going to cut it, but I found that it was just fine.
There were a few plateaus, those suck.
But, for the most part I just kept increasing my distance each month and trying to walk as fast as I could (go MP3 player with loud, fast dance music) and that worked out fine. I was very careful not to do anything that resulted in bounce.
SO I wore a job bra-(new balance makes the only one I ever found worth a shit-it clips around under the boobs after you put it on, so it can be loose enough to GET ON and then tight enough to stay put!). Even though the bra wasn't enough to be free to run, it did keep bounce down when walking fast. Big help.
I still wear a jog bra when I walk/run or workout, and now I'm only a B/C cup! But, it just does make a huge difference.