I'm assuming since you asked about the Garmin 301, that that is what you have, right? I have a 305 but I downloaded the owner's manual for the 301 and was reading it to figure out how to set average pace.
On page 41, it gives instructions for setting up Custom pages:
1. Press mode to access Menu mode. Then highlight Settings and press Enter.
2. Highlight the sport you want to set up a custom page for. (Are there other sports besides running?)
3. Highlight Set Custom Page and press enter.
4. Press enter to select the Custom 1 field. Select the type of data you want to display. This is where you would select Average pace.
5. Repeat step 4 fore the Custom 2 and Custom 3 fields. Of course, you can select ANY data you want for any of the fields.
When you are training, press the down or up arrow to select the page you want to view.
Hope that helps.