The Google Fit app is available on Android phones to keep track of your exercise and activity throughout the day. If you don’t see it installed, go to the Play Store to install it.
Setting Distance Units
The setting for distance units may show up as kilometers. Follow the steps below to set distance units for the Google Fit app.
STEP #1
Start at the main screen for Google Fit as shown below.

STEP #2
Tap on the Profile icon in the lower right corner. This will reveal the main Profile screen as shown below. Note that personal entries below have been hidden.

STEP #3
Tap on the gear icon in the upper right corner. This should reveal the Profile Settings options page as shown below.

STEP #4
Tap on Distance to reveal a pop-up selection menu as shown below. Then select Miles (or Kilometers) to set your preferred unit of measure.

STEP #5
Confirm your selection. You will be returned to the Settings page where your new Distance selection will appear as shown below.

Further Reading
Some people who have Android phones may not be aware that the operating system on their smartphone is called Android. Examples of Android phones would be smartphones such as the Samsung Galaxy, Motorola Moto, Google Pixel, LG, or some other brand and model.
This document refers to Android 8 – Oreo. This is the version currently installed on Moto G6 phones. At present, the current version is Android 9 – Pie. Presumably these instructions would work with some older or newer versions of Android.