So in two weeks I'm doing the Hy-Vee Triathlon. I've been training triathlon-specific for about three months and feel like I'm better shape than before.

I speculate I'll be able to finish in a little over four hours.
So now I'm thinking about what's next. After steeping myself in Lore of Running and Brain Training, I'm convinced that Tim Noakes and Matt Fitzgerald have the right idea. Although they focus solely on running, they stress that continuous training in one sport is the best way to improve your pace. In Fitzgerald's 10k plan, the runner goes 6 days a week running, with an easy cross-training workout after each run to recover. One day is reserved for nothing but rest.
So here's my multi-year "plan", if you can call it that. While cycling is considered the most important leg, my opinion is that since the race comes down to the 10k, more emphasis should be on running faster before tackling the other two.
Once I get up to a speed that I like (say, 40 minutes instead of 55 minutes for the 10k), I switch to one of the other sports as the primary focus, with a couple interval workouts per week to maintain the running speed
and the third sport being used for recovery. Once I hit a certain speed for that sport, I switch back to running and repeat. When I switch again, it's for the other sport, while being mindful of how well I'm feeling. If I need to pull back, I will pull back in intensity and rest a few days.
So this pattern would look like R -> S -> R -> B -> R -> S -> R -> etc, with each period lasting about four to five months. Between periods will be about a week of light exercise and rest. I'm hoping that with a such a schedule, I would be able to maintain the speeds I have achieved in the sports with less focus. If not, I would probably need to throw in biweekly workouts that would put more emphasis on them.