weight loss for my lab

Question
I have a 6 year old lab. she began to slowly gain weight about 3 years ago. i did not really notice until others started mentioning it. Last time she was weighed she was 109 lbs. My vet. said she should be between 80 to 90 lbs. I cut down her food from 4 cups to 3 and it did not seem to change. If I walk her too long,about 45 min. to an hour she gets a little bit of a limp in her back legs. I just moved by a lake and she loves swimming, after 2 hours or so I have to beg her to come home. It also does not make her limp. Is this the best type of exercise that I can offer? I've been doing this for almost 4 weeks now, 3 to 4 times a week, every other day. How much time will pass before I will see any improvement? If not what else should I try?

Answer
My lab was 80 pounds and lost her wasit line at 3 years old..had just turned when I started to noticed she was getting overweight, today she is 75 pounds and looks liks shes supposed to. All I did was feed Authoirty lite chicken based kibble(avalible only from petsmart stores) in the reccomended amount as apossed to leaving it down all the time, I walk her and my other dogs every morning for a brisk ten minute walk an they play together for a few hours everyday thats it its all I did and shes perfect condtion now. Dont walk her for so long if it is hurting her back legs, take her for a brisk 15/20 minute walk in the mornings or for two 10 minute walks daily you've not mentioned if she gets to play and run with other dogs so I'll assume she doenst, Swimming is an excelent exercise and about the best you can do for a water retreival dog..keep this up but if it helps makes the sessions shorter 20/30/60 minutes at a time but do it Daily. I reccomend a lite kibble during her weight loss program, lite kibbles are avalible in about every top end food out there. Best of luck.