That does not sound fun at all! My guess is you are holding tension in your ankle throughout your gait cycle, as well as (probably) reaching too far out in front of your center of gravity (and leading with your lower body instead of your upper body). This results in a hard landing on the back of your heel (versus a softer landing on the front of your heel, as taught in Chi Walking). And the probably the most important thing is you are probably not maintaing a nice tall spine and engaged core - understandable at mile 25, where you really need to be aware of that as you tire. We all tend to slump when we tire, and realigning the posture along the way is key to efficient movement.
So my suggestion would be 1) start working on correct alignment (you’ll get that info in your Chi Walking materials; you can check the YouTube channel chilivinginc for further tips) and 2) learn how to relax your ankles. As you get into correct alignment (tall spine, engaged low abs) your stride length will shorten a bit (this is a good thing) and you’ll land closer to your center of gravity, more on the front of your heel. The next step will be to learn to peel your feet up (rather than push) so that your ankle starts the gait cycle in a relaxed manner. Just let it stay relaxed as you walk. This will in turn relax your lower legs and give your calves and shins a rest.
For contrast, pull your toes up toward your shins and feel the tension. Now shake out your ankles and peel up starting at the heels, just letting your toes drop and relax. Notice the difference? Make a choice to start moving that way (it will take time to break old habits) and your legs will thank you for it.
Good luck and keep us posted!
Certified Chi Running and Chi Walking Master Instructor