Side effects of doing half of a "doubled" workout

Question

The P90X chest and back workout consists of a set of exercises which are done in two sets... do each exercise (pushups, pullups, rows, etc) for X seconds (varies for each exercise), and then repeat the whole thing again. It takes about an hour to complete.

I don't have a full hour to devote to the program, but I will be able to do just the "first half" each night. I remember reading that 80% of the muscle mass gain is acquired from the first set, and that repeated sets are mostly done to improve tone. So, my questions are:

1. Is this true?
2. Based on the answer to the first question, is it worth it to complete only half the prescribed regimen?

Answer 1

If you decided to just take the first half of the p90x program, and package it as a lighter program... let's call it p45x, and get people to do it, they'd still get more fit.

Regarding the claim "80% of the muscle mass gain is acquired from the first set, and that repeated sets are mostly done to improve tone", that shouldn't affect your decision. Mark Rippetoe says this about tone:

Most exercise programs that claim to improve muscle tone are actually lower-intensity hypertropy programs and are only moderately effective for improving muscle tone. If "tone" is the goal, strength is the method.

So, if 80% of the muscle mass gain is acquired from the first set, then that's contributing greatly to your tone. You shouldn't think about them as separate things.