Another New sneaker help for walking alot and running

RadishEater

I usually run between 5-6 miles per week and also a big walk/mini hike on saturdays netting another 4-7 miles. I'm just inching into the 300 miles covered mark if you count walking and running.

I've been using my running shoes with new extra cushion/arch inserts for both, which I bought in early March but I'm not only starting to worn down most of the tread at the sole, but also this week I have notice my shins just starting to ache. (I've had shin splints issues a long time ago), though that could be from walking too much in my heeled boots.
I'm planning on picking up some new inserts and doing some shin muscle strengthening exercises this week.

Does anyone have separate special running shoes vs walking trail shoes? Any good walking ones that last a while under 130$?
I really like my 880 New balances for running but I have never purchased walking shoes for trails before, so I don't know where to start looking.

Lastly, if you do have specific set of trail walking shoes about after how many miles/ months do you usually replace them?