Exercise is critical to your metabolic health. Not only does it increase insulin sensitivity (link), something as simple as a 20-minute walk after meals can blunt a glucose spike, avoiding that post-spike energy crash.
If you do not have other wearables or do not wish to use Apple Health or Google Fit, you can manually track events by following the instructions below.
Tap the plus sign at the bottom right, then "Event," then "Activity."
From here, you can select the type of workout - everything from walking to weightlifting - then just add the time and duration!