What to do about overweight dog in the desert?

We have a cavalier king charles spaniel who is almost 2yrs old and we live in Phoenix, AZ. She is about 7lbs overweight now due to being on medicine for chronic bronchitis. We try to take her on 2 walks a day but during the 115 degree summers, we have to make it limited. When we play indoors at her & start throwing her toys around she just lays there and looks at us like we're crazy. We just started limiting her food portions and eliminating treats. Based on her behavior and the climate, what do you suggest to get our dog to be more energetic and lose weight?