There is nothing quite like the unique rush of finishing a good run. Done right, there's not much of a mental letup on a run with the constant check on breathing and energy and slight adjustments to pace, form, stride length, and cadence with the ever changing terrain and elevations.
With biking the satisfaction comes from distance covered, time on bike, and the challenge of optimizing gearing and cadence to maximize speed and workout benefits.
The rush of swimming is about the technique, rhythm, the flow through water. Like running, you don't get the breaks, the coasting of biking, but with swimming you have to be much more aware of how much you can push it because it's a lot harder to recover during a swim than during a run. You can only back off so much!
I love the mix of the three sports, but I still see running as my core, my foundation. It's pure and minimalist - no pool, no bike required. And what a great run I had today!!