It's very easy to set up your Garmin Forerunner 235 or 230 GPS running watch to alert you to your preferred run/walk intervals. On your watch, the flow is:
Menu > Activity Settings > Alerts
If you haven't set anything up yet, select Add New and then Run/Walk. Follow the prompts to set your preferred time for your running interval and your walking interval. When you have these alerts turned on, your watch will notifiy you when it's time to switch between running and walking. You can have your watch give you a vibration alert, a sound alert, or both.
How to Set Your Interval Alerts to Sound or Vibration or Both
Menu > Settings > System > Sounds
You have a choice regarding how you are notified about your run/walk intervals: via sound or vibration or both. There are three settings to toggle here: Key Tones, Alert Tones, and Vibration. Alert Tones are a kind of "twinkling" sound to indicate each interval change. I have mine set to Vibration only and rarely miss an alert.
The reason I wanted to post this is because I often see runners asking on forums about how to set up more than 99 intervals on their Garmin. They are referring to a different kind of alert, for an interval workout, that is limited to repeating up to 99 times. Garmin lets you create this kind of workout (usually for speed) where you are prompted to run for a certain amount of time or distance and then rest for a time. These workout intervals can be repeated up to 99 times. When using the Alerts feature outlined above, the run/walk notifications will continue for the duration of your run.