Hey there RegarbC,
Thanks for reaching out to the Apple Support Communities. To get started, if you tap on the Activity widget on the Lock Screen are you able to see the correct information in the Fitness app?
If so, does the widget remain inaccurate after restarting your iPhone? If unsure, give that a try with the steps shown in this link here: Restart your iPhone - Apple Support
How to restart your iPhone X, 11, 12, 13, or later
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.
While restarting may seem like a simple step, it can often help with resolving some unexpected issues like this.
If still having issues after restarting, try removing and readding the widget to see if the issue persists: How to add and edit widgets on your iPhone - Apple Support
How to remove widgets
1. Touch and hold the widget that you want to remove.
2. Tap Remove Widget.
3. Tap Remove again to confirm.
Removing the widget can help refresh the information between the widget and app, and may help with resolving this issue.
Keep us updated on the results.
Cheers!