Ngala i started running when DD was 6 weeks old and my milk supply only started to drop off when she was 5 months old, due to my thyroid going crazy (i had a big goiter by then too too!), me taking the mini-pill, having to move house post-break-up with no help and a lot of emotional difficulties with my XP.

If you begin doing 40 minutes and build up to 90 you should be fine - i went through a break up when BFing DD and literally ate NOTHING for 5 days straight once and my milk was unaffected. Our bodies can cope with a lot but it's sensible not to put too much strain on when you can avoid it.

The one thing i would say though is that lactic acid can get into your milk after a workout. It's totally harmless but tastes different and some bubs don't like it. So feed, then work out, so there's a few hours before the next feed, and the LA will be gone by then. I had no worries with this. I found if i fed then ran after the run my boobies were still totally empty, but if i then put my feet up and had a big glass of milk or juice and a rest they were as full as usual by the time DD was hungry again. You'll see they're less full after a workout and feel bubs isn't satisfied after her feeds like she usually is if your supply drops. DOn't pay too much attention to the weight chart as lots of ubs drop a curve or 2 around 3-5 months. It's more than likely that your supply will be higher on days when you haven't been to the gym, which is fine, just feed her when she wants it (it might be handy to have some EBM you can offer if she's hungry when you still have lactic acid boobies) and for every gym session make sure you get good food (don't cut out to much carbs, try to eat less fat instead, carb-denial leaves you too knackered to workout and makes your body go for the easiest to access calories - your muscles) and good-quality rest (with your feet up, not running about crazy in the house but not counting it because you're in your PJ's still - we've all done it!) and all should be fine.

Best of luck!

Bx