It seems to me that you are undermedicated.
In the same way that you burn up energy as you exercise, PwP burn up dopamine as well.
To see if this is the case, time your route, noting down how long it is since you took your last levodopa medication. I suspect that if your walk is at the time you're fully on, perhaps 60 -120 minutes after you're last medication, you perform better than if you're "off".
Stick with it. I think walking is one of the best therapies that there is for PD.
I'm 11 years post diagnosis and until recently have never had any trouble walking. I often walk 10 miles plus in a day. I am now starting to get pain, due to a poor posture, caused by dystonia, straining my back.