Long-term shin pain

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I walk four-five times a week with a double stroller (about 120 pounds with the kids.) The terrain is mixed- sometimes dirt, sometimes pavement, and always with a lot of hills. I usually go for 60-90 minutes at a time. I make sure to stretch after for 10-15 minutes. This has been my routine for many months. I usually swim a couple times a week and do a small amount if strength training (not enough.)

Every single time I walk my shins hurt very badly. It's immediate and lasts the entire time. I've tried changing the way I'm walking, tried to make sure I wasn't pushing my stride out too far, and alternated between my New Balances and my Teva sandals. (I'm limited in wear due to bunionettes.) Nothing seems to be helping. I understand that likely the stroller and hills are complicating the situation, but I'd really like to step up my game and feel limited because of the pain.

Any thoughts on new/different attempts?