If that’s the case I would really recommend investing in one that straps your boobs up as closely as possible so there’s absolute minimum movement, after a couple of kilometres holding them to your chest will get pretty tedious and you’ll probably expend more energy that way (totally non-scientific guess there).
I’d say don’t spend less than about £30 on one. I’ve got a couple similar to this one from M&S: http://www.marksandspencer.com/extra-high-impact-underwired-serious-sports-bra-a-g/p/p22426653?image=SD_02_T33_6319_Y4_X_EC_90&color=BLACKMIX&prevPage=plp
They’re good because once they start slacking or you inevitably lose weight from all the running, you can make them into racerbacks which makes them tight again so they last a while.
A lot of people seem to swear by Shock Absorbers too, they’re about £40 for the ones made for running. Once my M&S ones have given up the ghost I’m going to buy one to try it out.
(btw sorry if this is all obvious and patronising and you’ve already got a good sports bra but hopefully it’s helpful)