Correct, although the 20 minutesx3 is a pop-fitness fiction. For general fitness purposes, an hour a day of hard aerobic exercise is pretty much the minimum. If you want to impact your weight without having to think much about counting calories, more is better. Lots more.
Fat is stored fuel. Getting rid of it is conceptually simple, not so easy in execution. Either take in less fuel, or burn more fuel than you take in via activity. Fat is a relatively efficient, dense method of storing excess fuel (calories), so its not something you can change quickly. If fat went away quickly and easily, we wouldn't be here because our ancestors would've starved to death during lean times.
A pound of fat is the equivalent of about 3500 calories. Walking is good, but its not the best. Figure roughly 100 calories burned per mile. More if you're larger, of course. Think back to basic physics... it takes more energy to move 200lbs than it does 120 lbs. A Ford Fiesta goes farther on a gallon of gas than a Cadillac Escalade. As you can see, that's an awful lot of walking to have any measurable effect. Again, walking is very good for a lot of reasons, if you devote sufficient time to it.
The best exercises in descending order of calories burned per hour:
1. Running
2. Cycling
3. swimming
I won't put numbers to it because there are so many other variables.
I could write a book...
One last note: Exercising a specific set of muscles or specific body part strengthens those muscles, but has no direct effect on the overlaying fat in that area. Cycling or squats in the weight room aren't going to do anything about fat on your thighs and butt. When you lose weight, the fat comes off more or less evenly from all over your body. You don't get to pick which fat goes first. It takes time. You can't expect to see any difference in days or even weeks.
OK, one more: If you have wider than average pelvic structure, there may be limits to what you can do. Mrs. Maxx has wide hips. She can starve and exercise down to 10% body fat, and they're still going to be wide. It tends to make her upper thighs look bigger than they are. (Fine with me, BTW)