You have to feed your dogs whatever you can afford and whatever works with them. That said, SD isn't generally considered a good food. Is it okay? Probably. Are there better diets out there? Definitely. Here's my favorite online resource for looking at food quality. It explains things in layman's terms. Dog Food Analysis - Reviews of kibble
How long and far you walk Peaches depends on her current level of physical conditioning I would imagine. You build up slow to these things. Most dogs can handle a nice walk, but in really cold or really hot weather, you don't want to throw them into the deep end right away. I have been walking with my dogs all year (I mean last year since this year is only a week+ old, LOL) and they are in good shape already so I feel confident taking them for a mile to two miles, although I rarely have the time to walk them two. People who are hardcore into conditioning will walk/jog/bike their dogs several miles, but there again, they don't just go from fat couch weight to extreme sports. They build up over a period of time.
As for the weight training, again, as long as she's healthy and built up to it, I don't see why you couldn't.