Don't let the name mislead you: a "stationary bike" is really NOTHING like riding a real bike. I've been training on a stationary bike all winter. Last night, I headed outside on my brand new bike and it was NOTHING like what I'd been doing all winter.
It was MUCH harder!
My hamstrings were all weak and wobbly. I only rode about 8 miles (the duathlon is 17 and the triathlon is 15) and I could barely walk when I got off...let alone RUN THREE MILES?! Oh dear, what I have I gotten myself into?