You can access any route that you create or star on Strava.com from the record screen or your Profile page in the Strava iOS app
To get to your routes, tap the 'Profile' button towards the bottom-right of the Strava app screen and scroll all the way down to select Routes (highlighted below.) From here you can browse the routes that you created or starred. You can choose to Use, Star, or Share the route. To follow your Route while recording an activity, select 'Use Route.' You will be redirected to the record screen automatically and the Strava mobile app will keep you on course.
You can select a Route to follow from the record screen as well. Select the Route icon (highlighted below) and chose from your list of Routes. Click to expand the Route you wish to follow and select 'Use Route.' You will be redirected to the recording screen where the Route is available to follow.
Stop your recording at any time to view the Route options. Click the Stop icon (highlighted below) to reveal the Route icon.
The “Route Back to Start” feature will plot the most efficient path back to the start of your activity. If need be, you can also switch or clear the route.
Learn more about Strava Routes