First, there is no "one" way to approach this. In the context of Body for Life, all the parts of the program (nutrition, weight training, cardio and rest) are meant to be done together as a complete program. When you start picking/choosing/altering different elements you will not get optimum results.
Something I get asked quite often is:"I'm doing Body for Life... but it's just not working for me." Invariably people with this question are not really following the program... they ignore the eating part, they do too much cardio, they don't train hard enough with the weights, etc. What I'm trying to get at is that this particular program (and it's certainly not the only way to approach things) is designed to be done "as is" for best results.
Now, to your specific question... there is really nothing "wrong" with adding intensity to your cardio; whether it's 3 or 4 times/week... or whether it's for 20 or 30 minutes. Basically, experiment a bit and see how your body reacts; also, see which approach you prefer. Better yet, vary your workouts... intense 30 minutes one day; less intense the next; something in between. Variety is a good thing... it keeps your body "guessing". Using your own energy levels as a guide can be helpful.
The big thing to watch for, especially with upping cardio intensity, is butnout. Basically, too much intense (which is very subjective) can lead to lean tissue loss and/or overuse injuries. But, for someone your age 30-40 minutes 4 times/week shouldn't be a huge problem... especially if you pay attention to your own energy levels (if you're having a low energy day, take a day off... or do a low intensity, leisurely session).
Anyway, treat it like an experiment (which is really the real key to developing an effective exercise program) by adding intense intervals to some of your workouts. Again, there is no "one" way that works for everyone. BFL is a very solid program, especially for beginners... but, there are tons of "tweaks" that can be done to make it better on an individual to individual basis.
Andrew
[This message has been edited by aml (edited 08-12-2001).]
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