You can start tracking from the menu, as long as navigation hasn’t been started yet.
So first, start the track recording, then start the navigation. At the end, don’t forget to stop the recording (after ending the navigation).

When you stop recording, you'll be able to save the track to your activities - the app will offer you the relevant options. This way, you’ll have a summary of how far you went, your speed, and so on.

You can also enable an option in the settings:
Application > Tracker button on map
That way, you’ll have a shortcut to start tracking easily - it's shown as a footprint icon in the top left corner of the map screen.