>Health officials recommend we get 30 minutes of aerobic activity a
>day, at least three times a week, right? Would the time vary depending
>on what kind of aerobic activity we do? For example, if I skip, would
>I still have to do it for 30 minutes to get the same benefit a 30-min.
>brisk would give me? Wouldn't skipping work my heart harder than a brisk
>walk, in less time?
Well, there's a couple of different levels of recommendation - one for health benefits and one for fitness benefits. The lower recommendations can help reduce your risk of disease, but probably won't do much to improve your fitness. The higher recommendations will help reduce disease risk and make you more fit. Which one is appropriate for you depends on your goals.
Both sets of recommendations take 3 variables into account: frequency (how often you exercise); intensity (how hard you exercise); and duration (how long your exercise session lasts). You are quite correct in that adjusting one of the variables (like performing higher intensity skipping instead of lower-intensity walking) affects the others (since you could burn the same number of total calories in less time skipping than walking).
For health, the recommendation is to get a minimum of 30 minutes a day of moderate intensity activity on most days of the week (preferably every day). This won't make you more fit, but it will get you out of the 25% of the population that is the least active and at the greatest risk of disease.
Moderate intensity activity would involve activities like raking leaves, walking your dog, vacuuming, mowing your lawn (push mower, not riding mower). Something that's going to be more vigorous than sitting around, but not so vigorous that you're really breaking a sweat.
For fitness, the recommendation is in the terms of the 3 variables:
Frequency - 3-5 days per week
Intensity - 60-90% of Max HR
Duration - 20 - 60 minutes per session (not including warmup/cooldown)
The intensity can be figured out by using your heart rate or your perceived exertion. This is a workload that's going to get your heart rate up, you may be a bit out of breath, you can talk but not sing, etc. The low end will be comparable to that "moderate intensity" above, whereas the high end will probably be something that will fatigue you in 20-30 minutes.
You want to fall into the ballpark with all three of the variables, but within the limits there's room for variation. So if you exercise at 80% of your max for 20 minutes on one day, if you're going to exercise another day at only 70% of your max you'd do it for longer to keep the workouts roughly equivalent.
>I think that 30 mins three times weekly is really a minimum. I also
>believe that they're talking about raising your heart rate to twice your
>resting rate and holding it there for 20 minutes. I'm not sure that
>walking would do that.
The ACSM criteria are specified above.
Walking can be vigorous exercise for many people, especially if they are new to exercise. As walkers become more conditioned, racewalking techniques can be employed to increase the intensity further. As walking speeds get above about 4-5 mph, walking becomes less efficient than running, and as such has a higher energy cost than running at the same speed - thus, walking can be a very effective mode of aerobic exercise.
Of course, we're not talking about an idle stroll, either. Maintaining a brisk pace is important if your goal is to use walking to improve fitness.
>If you could do more (either longer or more often), you'd be better off.
>I've also heard that in order to lose weight you need to do at least 300
>calories each time you work out (those three times a week).
In order to lose weight you need to burn more calories than you consume. 300 Calories per session is a reasonable amount for most people, but if you burn fewer calories in a session you can still lose weight if you're burning more calories throughout the day on average than you're taking in.