That the reason they want folks to exercise. Regardless of the fact that it's good for your joints, your circulatory system, and the old ticker, it helps to build muscle, and tone you, as you lose weight.
An hour at a time for at least five days a week sounds like a lot, but you don't have to do that hour all at one time. You could two two 1/2 hr. sessions a day, instead. One thing I've found that helps me, when I can't get out and do a long walk, is to walk in place, while I'm watching TV, instead of just sit there. I start with just some slow marching, then as my muscles get warm, I speed up a bit, adding knee lifts, stepping from side to side, then lifting my arms up to the side, and over my head. After my arms are warmed up, I pick up a couple of 1lb. weight balls, and lift using those. After 30 minutes, I do a five minute cool down, just slowing down the steps. Then I do my stretches, and I'm done. Good exercise, without even realizing I've done it! At first, you may gain a couple of pounds, but likely that's muscle, especially if you see your clothes fitting better, in spite of the weight gain. Eventually, the weight will start coming off again.
Must work, because I've lost 40 pounds since Christmas, and for almost half that time, I wasn't able to get outside and walk because of the weather, so I did the inside sessions.
When we finally clear a path to it, in the basement, I'm going to start using our Bowflex for strength training. ;o)