I once ran ~500 half-marathons in two years (non-competitive).
There's no such thing as overtraining; there's only under-recovery and a lack of balance with nutrition.
Getting away from cotton socks (wool or polyester) is the most common fix. Getting shoes that fit your feet well is another big one.
Also, are you walking a similar amount as where you came from? Changes here could impact walking form (i.e. forgetting how to walk).