How much exercise can a 15 month old Labrador have? 

Molly usually has about 1 to 2 hours in the morning, mostly running about playing off lead with her pals.  This tires her for a couple of hours.  I have just got back from the school run with her and decided to take the senic route home through the woods as the children enjoy it,  She probably had another hour of play with all the children.  Now that the weather is better I would like to walk home from school with her through the woods most days.  Is this going to be too much exercise.  She is very fit and lean and has had her exercise built up gradually but I dont want to overdo it.  She wouldn't get so much exercise at weekends so she would get a bit of a rest. 

Thanks for your thoughts.