i hand walk my minis for exercise every day, and it is an area zoned for horses, so its okay if they poop in the street. nobody cares, and it dries up and goes away eventually.
we have to walk them for exercise. i lunge occassionally, just to let Ghost run, but we mostly walk because i can't lunge the others (different reasons for each, but none of the others would do so well) so we go for walks all around the neighborhood for exercise and the neighbors just love them! walk a few steps, stop, pet, eat grass, walk some more.....etc.
we have fun. it seems to be enough for them, we all enjoy it. sometimes we only take one, sometimes its a family affair and mom and dad come with so we get them all out! :bgrin
just walking should be fine, if she is a little chubby, see if you could lunge her somewhere. Ghost (the only one i ever lunge) does it about once a week, and she's fine. but she really loves to just be free and run. we don't have an arena to run in, but we take her to another barn sometimes and let her go in there. she loooooooooooooves it. i'll have to take the others there soon (just got them last week, so we'll go maybe this weekend.) here's ghost playing in the arena. sorry if they're a little dark.