I'm wondering about this too. It was easy enough for me to go to the gym for an hour and walk for an hour everyday the last time because ds had two hour long naps a day (although it was a bit stressful losing my housework / relax time in the day). I didn't just walk for walking's sake, though. I had a rule that I would never drive or take the bus anywhere if it was physically possible to walk it, so it was all running errands, going to see friends, shopping...etc.
I can't do either of those things now as ds is 2, doesn't sleep so much in the day, doesn't want to be in his pushchair for more than five minutes and demands more time and energy than he did a year ago (although not, unfortunately, aerobic energy as he walks too slowly!)
I am contemplating doing exercise videos at home while ds has his 1 sleep a day, but am already dubious about how it will work as motivating myself to jump up and down in my living room where I am used to relaxing, drinking tea and watching telly, will be more difficult than getting my sorry arse to the gym. I am also less likely to do that on days dh is home from work. He would find it all far too hilarious.
This is my biggest problem as the only way I have ever managed to lose weight is by exercising consistently. If I don't have those lovely endorphines I just lose the will to starve myself!
MLB - 15,000 is brilliant. That would easily make a difference. The main thing is how it makes you feel. I think exercise makes you feel energised and positive and that wins half the battle for you.
Here's me making it sound easy when I've put on a stone in no time at all and haven't been able to get the motivation up to do anything about it. It is the hardest thing in the bloody world!