Hmm, Zee at 9 months is over-exercised then according to those guidelines.
I have walked her less than I would Remus who is 2 yrs and he gets 6-8 miles a day and some of that I run with him (no running with Zee).
I cannot logistically walk them separately unless my OH is home, so I've reduced Remus's walk duration. We do 30-40 mins 3 x per day. The first walk is a lot of off-lead, 2nd and 3rd ones are 50/50 lead/off
How can you tell they're getting too much exercise, physically? Zee never stops, she's like a duracell dog.