If your dog isn't used to getting many walks start slow and stop if she is looking fatigued or panting excessively. Other than that, just train her like you would yourself when working up to longer distances. An adult lab could go for miles a day
Considering breed, health and age I see no restrictions on how much activity she gets. I would highly suggest giving vitamin E and an omega 3 fatty acid supplement along side a glucosamine/chondroitin supplement to keep those joints happy and healthy. You want to prevent arthritis as much as possible!
I think with a lab, I would reccomend at least an hour of exercise a day due to high energy level, of course that doesent mean letting the dog run around the backyard for an hour, mental stimulation is very important, regular walks, playing with other dogs, running off leash will keep the dog in great shape.
I have a 67 lb , 4 year old weimaraner, we do a 4 mile jog in the morning, then I get home at 5 and let him off leash to play with neighbors dog for 15-20 mins, then we go for another 2 mile walk in the evening.
On the weekends we usually go on at least a 5 mile hike, and I let him run around and play with other dogs at the dog park for at least an hour, depending on temperature. In the summer, I pack up my kayak and we head out to the lake, he loves to accompany me on my kayaking trips and then I let him swim around to his hearts content.
But keeping my dog fit and healthy is not only beneficial to his health but to mine as well, I would be a total couch potato if I didnt have a dog.
I have the two labs. My yellow is around now 85 lbs. and my chocolate is around 72 lbs. I take them on 3 mile walks and I do try to get these in everyday with some day two walks. Today its raining on and off we diligently went for the walk and cut it short got soaked (fun times)! But when I started to really get into the walking because my chocolate was 88 lbs. way overweight Oh yeah did I mention they have an OCD for food heehee~anyway she is now down to 72 lbs and still going (yeah) ~my chocolate lab was not use to the rigorous routine of walking so much and we started out short then increased the walks as she got more use to them! She actually when we started was not use to it threw up which was not good at all. We slowly then went for short walks then increased and now she can handle the walks and like I said two at times! so go slow then increase. The more stimulated they are the better they are! There are some days like I should have not today you do miss and gee that's ok! Labs do need to be walked and played with they cant get bored! I have a pool in the back yard they swim in and a box of toys inside and outside so lots of stimulation. I am considering getting the agility toys for them well all the dogs I should say but we shall see with he nicer weather coming! But with walks, swimming playing yep they do get their exercise! And they love it they cant get enough!
I walk my dog Seamus at least once a day (he's a standard poodle). Seamus is about 3 years old. While on the subject of walking, I take him probably 4 or 5 times weekly to Home Depot to walk. Everyone there knows him by name, he is referred to as the Kalispell Home Depot mascot.
Other times I'll walk him outside, just depends on what I feel like.
In Home Depot, the walks are shorter, we maybe walk for 30-45 minutes at a really fast pace. We probably do 2-3 miles.
Outside, we will go walking for 5-6 miles some evenings.
An interesting thing is that he is *just* as tired from our shorter walks in Home Depot. That's because he has to use his mind, I'm constantly turning to go down different aisles, or sometimes I'll just turn on my heel and go the other direction. He has to pay attention to me or I'll walk into him.
Walking in Home Depot has been great for his socialization and just plain getting used to things. Sometimes we'll walk by the big circular saw when it's screaming making a cut. Seamus doesn't even bat an eye.