You can upload an activity to Strava even if wasn't recorded with GPS.
To enter a manual activity, select the plus sign icon on the bottom right of your feed. You will see options to add a manual activity or record an activity.
From the manual activity edit screen, you can enter your time, distance, and speed. Add a photo, select your activity type and a tag. Finally, set your start time, attach gear if applicable and add a description. Once you save the activity it will be uploaded to your Strava account.