Kudos to you for having the tenacity to continue your exercise over such a long period and be consistent about it.
I see that you seem to have had no problems in the past but I must tell you that you might be overdoing the quantity a bit. However if you haven't had any problems so far, it may be unlikely that any occur now. Some precautions to keep in mind:
Wear comfortable shoes that take the impact of the walk and reduce risk of injury.
Walk on a soft-ish surface so as to reduce the impact on the joints.
Maybe you could take on an alternate option once in a while like cycling, swimming or add some other form of exercise that completes the fitness triangle.