I am a 5'7, 125 lb very lean, fit women. I run 5 miles a day and have done numerous half marathons. Would this affect results?

Breast fat transfer is a great tool to balance breasts, improve symmetry, and gain up to one cup size in volume.  In general, 200 cc per breast is typically the upper limit of fat injection recommended by most plastic surgeons.  It is not that more cannot be injected; but on average, this has been determined to be the most that can be expected to live during one transfer.  Multiple transfers would be required for additional cc's.  In general, the abdomen and thighs are preferred donor sites but fat from other areas can be used.
Some of the retention rate will depend upon the amount of fat transferred, as smaller volumes will likely have a higher survival.  Occasionally, a patient can resorb a great deal of the fat despite optimal transfer procedures. I typically transfer from 250 cc to 400 cc or so per breast.  Thinner patients are outstanding candidates in my opinion, because the fat transferred is extremely resistant.  
Results are largely dependent upon the skill of the surgeon, his/her ability to remove fat without creating contour deformities, and your expectations. You would need a consultation with a board certified PS to determine your available fat stores.

If you are lean and running marathons, a fat transfer may not work well. First, you do need donor fat to transfer. Second, with the movement your breasts do while running over 26 miles, the fat may be absorbed more quickly. I would suggest considering implants. Long term you will have a more stable look and the implants will hold up for marathons.