There is evidence that only extreme exercsie affects your BF - runs of more than an hour
I was walking a few miles every day at 4 weeks and running at 6 weeks - but only about 30 mins because I had 2 in the jogging stroller.
By about 4 months I had proved to myself I was fit and retreated to the sofa for the next year as DS2 was a shockingly bad sleeper