That depends on your age, weight and height really. However with a low carb high fat diet (there are others that would work too of course) followed to the letter and 3 exercise sessions a week you could lose 6-10lbs and possibly more.
It sounds like you're already fit and healthy, so it's vanity pounds you're looking to lose. Which is fine, it's doable.
You should be able to lose half a stone, maybe even a stone, in 8 weeks. You are very unlikely to lose the 2 stone to take you to 7.5.
When you walk the dog, could you it trainers on and try for running? If not, imagine you're late and walk really fast. The idea is to get your heart rate up, so your breathing gets laboured.
Also up the distance of the dog walk. It's approximately 1000 steps for 1k. So a 5k walk/run would be good for calorie burn.
Then try to eat low calorie foods. Have at least half your plate as above-ground veg every meal. And make the other half low fat protein/meat. Avoid cards (bread, pasta, rice, oats) and anything with sugar in.