It's amazing how much a little weight can affect them. Lemon gained a pound and a half over the winter. She still had lots of energy, but she wasn't pugtonaing like she does now that the weight is off.
When it's cool out, we go on hour-long walks at a fast pace, mixed up with trips to the park. But in the summer, I'm really careful. If it's over 80°, we're not out for more than 15 minutes. She gets her exercise inside on those days, with lots of fetch and tug. Oh, and she eats no more than 1/2 - 2/3 cup a day.