Try this, it really does works!
The easiest and natural thing to do is to walk. So, choose a time of day when you are not busy and make that your walking moment. You do not have to power walk, but it must be continuous. That means every single day. No matter if you are on vacation or not. There is always walking to do. Start out going for a walk around your block for a week. Then the second week if you feel comfortable doing that then increase it to two blocks. Do not try to do more until you are comfortable walking two blocks. By the end of one month you will probably be at a comfort level where you can now walk for one hour up and one hour back to your home without getting winded, as long as you leisurely walk.
As I have mentioned before, do not power walk. Just leisurely walk non-stop every day. That includes in the rain and the snow and the heat. The key is to prepare yourself for the walk. If it is raining cats and dogs, don't bother with an umbrella. Get yourself a hooded raincoat and water galoshes. If it is snowing outside then buy a warm coat and scarf and gloves and boots. You might want to get a pair of dainty long johns and thermal socks. If it is hot outside, buy a headband and clothing that flecks the moisture away from your body.
You are not going to die of thirst in an hour at a leisurely pace so forget about the water bottle. Drink your water and empty your bladder before you leave the house, so that you will not be uncomfortable during your walk. Well, there you have it. One hour up and one hour back. No more, no less. Do not stop to pet the dog or smell the roses. Don't even stop to talk to your neighbors. Just smile and say hello and goodbye as you continue on your way. People will by now have noticed you have begun to slim down and will probably want to walk with you. It is alright to let them. But, stipulate to them that they must walk at YOUR PACE and on YOUR CHOSEN PATH! Resist the urge to power walk!
By now you are admiring yourself differently and have made walking a daily routine and it didn't cost you a thing. After you have been doing this for a good six months, it is well within reason that you want to do more for a variety. Well, good for you. Get out that ratty blanket that you keep on your bed and throw it on the floor. Now do get down there and strive to do five situps. Stop! I said 5! Only do 5. There's no race here. You are not in competition. Continue to only do five during the time that you have chosen to do them for one month. The second month if you choose to do so you may increase your situps to ten times each day. Continue to increase your situps ever every month until you reach fifty. Yes, I said 50! I know that you know someone who can do one hundred or more. But, you are not them and you asked for my advice. The reason that I make fifty a cut off point is because you want to make this a lifetime routine coupled with your walks and you don't want to risk hurting yourself or becoming bored.
As you do your walking and situp routines, don't forget to have fun.