As yes, that was me.... and what a lovely peachy dewy face I have since ditching the 10ks. Well perhaps not quite, but I'm definitely looking younger and softer round the edges, which is no bad thing imo.
Speaking as a 3 week into the trauma that is new motherhood mum the best advice I can give you on post natal exercise is get a good buggy, comfy sling and pair of walking boots. If like me you choose to breast feed it is fairly safe to say that you will spend every minute of your day either feeding, or settling the baby, or rushing about madly trying to do the 5 millions jobs around the house that will have inexplicably accumulated whilst you have been tied to the sofa with a baby latched on. Exercise routines do rather sadly join the ranks of things that just don't happen (like having a wee when you need one and getting a hair cut). And that is before you hit the dreaded growth spurt weeks. Like why does no one tell you about those??? I had NO idea, none at all, that a small piglet of a baby could eat quite so much, quite so often without either bursting or being sick. It's staggering.
But back to the exercise, walking is great, mainly because it will send a windy colicky over indulged new born to sleep in a nano-second (when hours of bouncing them in your arms has zero effect other than to cause a copious stream of projectile vomit down your back). I've worn quite a grove in the 2 mile circuit to the village and back and I'm only 3 weeks in. By 3 months I'm predicting a major rut and possible need for road resurfacing.
Enjoy the early days though and don't worry too much about fitness. You'll lose the baby weight (if you have gained any) in seconds, as you won't always have time to eat, your arms will tone up quite fast with all the lifting, and there is nothing like pushing half a ton of over fed baby up a hill in a buggy for lifting and firming the ar5e. It's a great life 
. I'm loving it 