There use to be a link for training plans that you could enter a start date and activity type that would go to the calendar. Where is that now? It had 5k and century rides on it.
If you are looking at the modern Garmin Connect, click the icon with the three horizontal white bars. Then click the words Training Plans. If you don't already have plans tagged as favorites, you should see a plan selection page. Otherwise, you can go here.
