Hi Eliza,

I used to work out for about 20 to 30 minutes too. Then I went to get a physical and my doctor said that was not a long enough workout. He said that you don't really start burning any significant calories until about the 20 minute mark. So, just when you're starting to burn some calories, your workout is over. Since this conversation, I have upped my workouts to at least 40 minutes, but they are usually an hour. I don't know how firm your tush is but if it's mostly fat, you should probably increase the length and up the intensity of your cardio workout to burn some more fat and calories. If you're stretched for time, try walking more throughout the day (do you have a pedometer?) or walking 20 minutes in the morning and 20 minutes in the afternoon.

As far as toning. You will probably need to do a bit more than walking to tone your body so here are a few weight workouts that I've done and recommend. Tom Holland's Total Body Workout 1, Crunch: Total Resculpt, and Women's Health: Train for your Body Type (this one also has a cardio interval workout). I don't know your fitness level but they are all beginner to intermediate level workouts so just do what you can if have a hard time with it at first. Alternating cardio with weight training days is a good way to start.

Monday- Cardio
Tues- Lower
Wed- Cardio
Thurs-upper
Fri- Cardio

Or if you want to do more weight training than cardio, just start the week off with a weight workout-

Monday- Lower
Tues- Cardio
We- Upper
Th- Cardio-
Fri- Lower

If you get ambitious and want to do cardio and weights on the same day, do the weights before the cardio. Good Luck

You need cardio and strength training; they complement each other.
Cardio is any activity that raises your heart rate. Walking for 20minutes just won't do it. Even jogging for 20 minutes is not that great, but it's better than the walking. If you want to stick with the walking, try getting some ankle weights so that each step is a bit more of a workout.
The minimum for cardio should be 30-40 minutes and that should be intense enough to get you sweating.

If you are near a gym, it may be a good idea to go for a session with a trainer so that you can find out where you are weak and to get a handful of exercises to get you started. There are many gyms that do not require a membership to workout. Find one where you can pay as you go.

If you don't want to go the gym way, search online for full body workouts. It may be easier to get started if you are doing the same exercises daily in addition to the cardio.
A great cardio workout is spinning. See if you can try out a spinning class; it is a great full body workout and also will be great for your legs and those glutes that you want to decrease.
Another option would be to try yoga or pilates.

You really need to do both because females lose the density in their bones as they age, and the strength training helps to slow down the losses.