Pain when walking & bra question

So, I may have overdone it today. Walked over 6 miles. I've been wearing a belly band, but still having a lot of pain with baby pushing down & into my sides, etc. having to walk incredibly slow...
My question is--how the heck are we supposed to stay in shape final weeks of pregnancy if every step is painful??
On an unrelated note, how do you get measured for a nursing bra? I've been wearing a medium Motherhood Maternity nursing bra since basically first trimester (my ladies immediately just grew about 2 cup sizes). I am roughly 32 weeks I just get a large in anticipation of milk coming in post-labor?