Except then the biker has 1 hour 40 minutes to take it easy, compared to the guy who walks who has to keep walking during that time, so it should even out. How tired are you an hour and a half after your bike ride, compared to when you finally arrive at your destination after a long walk?
However, currently walking does not make you more tired at all, so it just adds an extra penalty to the bike for no reason. So if you added walking (actually running since you move at max speed at all time which is a little weird) making you more tired as well as riding a bike, then I would agree, The bike should drain your energy quicker (but more efficiently) then walking.
Really, the problem is that right now there is no physical fatigue stat/meter at all - tiredness is more of a mental fatigue "sleep meter" that (mostly) only increases as time passes and (mostly) only decreases by sleeping. It would be strange if fighting zombies constantly for 12 hours straight with a screwdriver didn't make you at all tired, but riding a bike for an hour made you collapse in a heap.
Also, as a side note, it's not really fair to say there's a "degree" of higher efficiency. Bicycles are the most energy efficient form of transport in existence, and takes roughly one-third as much energy as walking does (depending on speed, incline, etc, of course).