Want to help with friends overweight dog...?

I took my friends dog out today and for the first time realised how bad she is in terms of her weight and attitude. She is overweight, doesnt really run or want to play and seemed to get tired real quick. I dont want to berate my friend but I do want to help the dog! We walked for two hours and she was wagging her tail the entire time -she just ambled along sniffing everything in sight and occasionally ran in front of me. I can take her out 2-3 times a week and want to make it count. Any kind suggestions? Thanks