Map my run is a bit long winded tbh.
Try www.bikehike.co.uk, it's easy to work out how to plot a route.
Plug in your garmin, and when you're happy with your route, click on "Download route"
a little box opens, check the gps unit box and click download route, it will then go straight to your garmin without having to save the file on your pc and import it from there.