In reply to goldmember:
As above. I have a 310XT and you can do this on the watch, but it is a bit of a faff - I find it much easier to do this in Garmin Connect and then upload to the watch.
In Garmin Connect, find the Workouts page (connect.garmin.com/modern/workouts). In the top left, there is a Create a Workout button. You need to select whether it is a running, cycling or other workout from the dropdown box next to the button, then click the button. Then on the next page you can set it up how you want. Once you have saved your workout, it will appear in a list on the Workouts page. For each workout, there is an icon that looks like two arrows pointing in opposite directions. Click this to send the workout to your watch. It will transfer via your ANT stick, just like when you send data from the watch to Garmin Connect.