Hello Bob's Mom,
It's very important to measure your walks appropriately, so we'd love to help. As with all unexpected iPhone behavior, try taking the following steps which may help with this: restart your iPhone and make sure it is up to date.
Restart your iPhone
“1. Press and hold either volume button and the side button until the power-off slider appears.
2. Drag the slider, then wait 30 seconds for your device to turn off. If your device is frozen or unresponsive, force restart your device.
3. To turn your device back on, press and hold the side button (on the right side of your iPhone) until you see the Apple logo."
Update your iPhone or iPad
“1. Back up your device using iCloud or your computer.
2. Plug your device into power and connect to the internet with Wi-Fi.
3. Go to Settings > General, then tap Software Update.
4. If you see more than one software update option available, choose the one that you want to install.
5. Tap Install Now. If you see Download and Install instead, tap it to download the update, enter your passcode, then tap Install Now."
Once those steps are taken, please test it out with a walk. We hope this helps. Let us know if we can assist with anything else at all. Take care.