Go down lots of hills to get used to the constant pedalling. Once you're used to fixed gear with foot retention, then you have the option to go clipless.

If you're going go with clipless, work on your pedalling technique by riding on a flat land and last 2-3 miles with just pulling up only, no down strokes, then rest, then pull up again for 5 miles, then rest, more pulling up for 7 miles and on and on and on.

Also, bomb lots and lots of hills with a low ratio (~65GI) and stay on top of your pedal strokes and don't ever fall behind. Climb lots and lots of hills by pulling up.