Another exercise support thread

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BH, another question. What if I do 10 minutes of cardio here, 10 there...is that as effective as just 20 minutes straight?

Does a long walk (like an hour) do as much as 20 minutes of vigorous exercise on the bike?

I do not know how much cardio I should be doing to get firmed up. I have been doing 20 minutes about 3-4x/week.