how much exercise lab puppy

Question
isabelle is 8 months old. she used to have a brother who was her playmate up until two months ago. we live on a mountain and we used to hike it until the summer. she would run around crazy ambushing and cooling off in mud puddles. now it is too overgrown to use.
sometimes she has to stay on the (pretty big) porch (with toys and nice bed) when i go out.
her brother cabin was obsessive about fetching, so he got loads of exercise. too much actually. so now im not sure with her how much is too much and how much not enough? she can go outside quite a bit, big yard, but she generally likes to stay close to me or play keep away. maybe 15 minutes at hi-speed and then she rests when she's pooped. im an anxious person, that's the real problem. always worried that she's not getting enough. she doesn't get walked on a leash.  when she plays or is getting ready to eat she defies gravity. she's always so psyched. inside we let her leap gazelle-style over the couch and play alittle chase, but it's never for very long periods of time. im so used to cabin (her bro) being ON 24/7 , i just need to know everythings cool.
thanks so much,
donna christner  

Answer
Hello Donna and thank you for your questions,

I'd say 30 minutes of fetch/running/tug of war or other active games would be fine for her per day, It would be really nice if you could still keep in contact with her brother or anothert dog of simaler size for her to play with now and then because tehre isno better exercsie or fun then two puppies playing together.