What fitness routine do you recommend for someone like me? please help?

i am very small in size about a UK size 10 but have 32G breast. im wanting to try loosing a couple of cup sizes without surgery. ive been told that with regular exercises and a good diet that this is possible but im unsure what to do. im not a gym girl or a swimmer. i do love running and work out videos but the strain my breasts are putting on my body is causing me to stop after 20 mins. i really want to power through and know that the more i exercises and work on my back i will be able to do better. so what fitness routine do you think i should do? just a weekly routine would be great with lots of cardio but not so much hardcore so i hurt my back? please help thanks xx