If I wanted to make my biking harder first thing I'd do is eliminate all the maintenance hassles:
-Switch to airless tires.
-Stiff heavy rims with stiff heavy spokes.
-Indestructible steel frame.
-Drum brakes front and rear.
-Fixed or single speed gearing.
-Dynamo to charge electronics.
-Recumbent seat for comfort (maybe a recumbent trike?).
-Roof for weather protection.
If that still doesn't make it hard enough then just adjust the brakes until they rub and if necessary just ride with the brakes fully engaged all the time. However, if you try this then you'll definitely need drum brakes because rim/disc brake pads don't last long enough.
Last edited by chucky; 10-10-11 at 02:13 PM.