Summerrain, I'd suggest that maybe its not entirely down to breastfeeding, but that maybe the fact that you have a small baby and therefore not long ago gave birth might also be affecting your ability to get back running the way you were before.
Important to remember that pregnancy changes your body dramatically and that you need to give it time to recover. 6 months post pregnancy and still feeding, your hormone levels will still be elevated and the looseness in your joints, caused by the relaxin your body made when pregnant (so that the ligaments of your pelvis could relax enough to allow your child to be delivered) is probably still evident.
Just because paula Radcliffe was back to proper running almsot immediately after her child was born does not mean that you should presume to be able to do the same. If you are finding it hard, it must be for a reason. Give yourself a break and take it easy while you are still feeding. Once your child is eating more solids and needs less nutrition from you (so when breastfeeding is more of a drink/snack than main food source) your body will adapt and you'll probably find you have plenty more energy.