Its hard to say how much weight youll lose in any amount of time. It all depends on your eating habits and the intensity of your workouts.
The plateau sounds like your body has gotten used to a certain routine and youre not mixing it up. Go with the walking routine I mention later and youll get your heart rate up and the calories will burn! Also, read my eating tips. Youll go a log way with them! Ive helped lots and lots of women (and young ladies) lose weight by giving them the same advice.
As for your knee....I know what youre talking about there as I have a bad left knee and it hurts for a bit when I start walking, but goes away after about 30 min.
I would cut out the spaghetti, and the sauce. If you want pasta, eat whole wheat pasta, and if you want sauce, either find a low cal, or just use it sparingly since there is a lot of sugar in it. I would leave the cherries alone too. You dont want to over do it on fruit. Some is good, but not too much. Natural sugar can be just as damaging. Apples are great for fiber (1 a day), and oranges and grapefruit are good in moderation. Try to stay away from certain vegetables too. Carrots are high in sugar, as are peas, and corn. Vegetables are important for fiber. Califlower, brocolli, celery, salads (light dressing and only a couple tablespoons), tomatos, cucumbers (in moderation), bean sprouts, cabbage, peppers, asparagus, and a few others are great for filling you up. At your age, you need some carbs, do go with brown rice (1/2 cup), baked potatos would be good for you (no butter), yams, and whole grain bread (not whole wheat). You drink lots of water, and thats great!
Your breakfast is high in sugar (youre eating mostly sugar products in the things you mentioned) and you should stick to things like bran flakes, Special K, mini wheats or the full size (not frosted). Plain oatmeal (with sugar substitute) is the best. Choose a fat-free or 2% milk for your cereal. You can also do a plain whole grain bagel for breakfast (no butter or anything on it...yes, just dry and boring). Also, you can eat egg-whites (they sell them at every store) and make omelettes with salsa and a little cheese. Lots of protein, low carbs, yummy and filling! You can eat this anytime...I do!!
The lean steak and chicken breast are great meals, but I would cut out the ham (too much sodium and its not very lean) and limit your cheese (dairy) intake. Eat lean turkey or chicken breast (deli style, not the processed garbage). If you like fish....eat it! Especially salmon. Salmon is the best to eat! Tuna is second best. Too many others are high in fat.
Once youre on your weight loss trail, and its working out good for you with progress, you can introduce the things you previously mentioned (the stuff I said to stay away from) back into your diet.
I eat very strict. Once I have good losses, I have a treat that I really crave or want. Then, I go back into my routine. Yes, its tough eating the same things all of the time, but its worth it in the end. Its important to stay diciplined.
You have that weight lost far sooner than the time period you are mentioning. In that kind of time, it sounds like a "cheaters schedule". Thats where you eat fairly well, but you eat things you arent supposed to, and then lose a little here and there, but gain here and there too. Go for broke! Get diciplined, stay focused, and dont stray from eating like youre supposed to. The exercising is important. If you can do your walks at a fast pace (to the point of heavy breathing) youll really get results. Get your heart rate up. Walk really fast for 2 minutes (time yourself), then, walk at a normal pace for 2 minutes, then, go back to fast again for 2 minutes. Keep this up everytime you go for a walk. make yourself tired! The sit ups dont do much except build your muscles under the layer of fat that is your stomach. Sit ups dont burn fat around your stomach. Push ups are great for muscle building on your chest, shoulders, triceps, upper back, biceps, and traps. Not great for burning fat either. Jumping jacks are good because you are moving.
So, just stick with everything you are doing. Youll be fine. Pay attention to what I said about what you should and shouldnt be eating. The things I mentioned will work great if you follow it, and the foods I mentioned will really help with the weight loss. Taking vitamins wouldnt hurt either ;-)