Exersising my lab

Question
I have a chocolate lab who just turned one year old.  I am in a dilema when it comes to giving him enough exercise.  I live in a residential neighborhood where dogs must be kept on leashes in parks.  I am quite a distance from the towns only dog park, that I am unable to get there every day.  How much walking on a leash is enough and is playing fetch in our small back yard every day enough off leash time for him?>

Answer
15-20 minutes of vigorous fetch is likely better than an hour walk.  If you can make it to the dog park a couple of times a week, thjat would be good too.  

If he is at home by himself all day, having a professional dog walker give him a break would help too.