Exercise regime for a 10 year old Labrador

Question
How long a walk should a dog of this age have a day. She has some hip trouble - occasionally limps for a moment or two - so the problem is whether it is better for her to keep active  or rest more. At the moment I take her for 40 minutes, we are not walking continuously or briskly.

Answer
There is no set time.  With an older adult, you can do what they are up for.  

For hip trouble on a senior, you need to make sure she is at the lowest good weight she can be, supplement with glucosamine and chondrotin, and keep her active with low stress activity (walk or swimming is good).  

40 minutes sounds fine if it's a leisurely pace.