Thought catching 'em all was hard in previous Pokemon games? ONM reports that you'll have to walk 1,000 miles to unlock all the routes on the Pokéwalker accessory that comes with HeartGold/SoulSilver.
The Pokéwalker is a pedometer-style accessory. You can store Pokémon on it and as you walk around, you'll build up watts that not only make your Pokémon more powerful but help you unlock new routes.
Assuming you take 2000 steps per mile, you'd have to walk 1,000 miles to unlock all the routes on the Walker.
Or you can just strap it to your cat.