Mazarin
I dunno what kind of dog you've already got, but I made one of my two dogs my official *exercise buddy*. He's a staffie - so a strong, athletic dog (not so easy with a Yorkshire Terrier or one of them other tiny doggies, I know!) and last year, despite having 60 min walks most days - he was looking flabby. So I took up running, and when I walked with him, walked harder, faster and over rougher ground (There are no hills here in North Yorks, so it's all on the flat, I'm afraid!)
I did the programme here:
www.girlsruntoo.co.uk
So did my staffie (although he's no girly!) Now he is svelte and muscly rather than knackered and pudgy - and I lost 30lb too! I dunno what your existing doggy is, but I'd recommend a really active breed for No 2 - as they can help you in your weight loss campaign.
Where I live is remote and there are lots of creepy woods, etc we pass and we can be out for 2 hours and not pass a living soul... so I felt a lot safer having a *scary* looking breed of dog with me on my walks (Not that he is scary!)
I've seen people doing that flyball training with dogs and that would probably keep you fit too, if you wanted to do it! (My dog's too thick to cope).
That's the best incentive I've heard for losing 20lb... By the way, I dunno where you live but if your present doggy isn't very young/active whatever, you can always volunteer at a local dog's home to walk the dogs whilst you're waiting for your new arrival!
PhatP x