(from Ken Raich email@example.com)
There are TWO distinct thoughts on this issue: Both present fairly reasonable arguments.
The real bottom line is that if you exercise for 90 minutes a day, you're interested in more than just fitness(don't get me wrong, this is perfectly OK). To stay in reasonable cardiovascular shape, you need to perform aerobically for 20-30 minutes at least 3 times a week. If you wish to be in better than "the minimum acceptable" shape, remember that the returns are not geometric (you won't be in twice as good shape if you workout for twice as long). Therefore, if you're going to workout for 90 minutes a day, splitting the time between one or two sessions probably doesn't make a significant fitness-level difference.
This is part of the misc.fitness.aerobics Frequently Asked Questions page (FAQ).
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