I don't think you need to worry about doing 2 sets of anerobic intervals a week at age 30. If that were a problem, most of my friends would have long since been dead.

The fact that your resting heart rate is lower means you're getting fitter.

There's good data that the bigger the spread between your max HR and your resting HR, and the faster the rate at which your HR recovers from a max effort, the greater your life expectancy.

My point, and I think Zinn's point, is that you can get a good cardiovascular fitness from moderate exercise, and taking it to the level necessary to compete doesn't advance your overall health once you hit your 50's.