You don't have to focus on the numbers, but goals will definitely help. You could try exercise or fitness based or health goals. They don't just have to be around the numbers on the scales. For example,

Drink 8 glasses or 2 litres of water each day for the next 3 months.
Eat 2 pieces of fruit and 5 serves of veggies every day.
Reduce the amount of junk food, limit naughty food to once a week only for three months.
Swim twice a week.
Swim 1km without stopping by x date

Set them according to your fitness levels and where you think you'd like to be at. Instead of weighing yourself, you could take your chest, waist and hip measurements once a month. That will give you an idea but you'll notice it anyway when your clothes start to feel looser

As for low impact exercise, I second the swimming. Gentle walking is also good (not sure how you are with that and your back though). Pilates is an excellent low impact exercise for people with back injuries. Make sure you find a proper studio and try and do the reformer pilates rather than mat work.

You might want to PM Mother Goose. She has a back injury and is very inspiring and motivating (she's been whipping me into shape LOL). So she could help you out with some back friendly exercises. Come check out the Fat Chic Slim page on facebook as well, lots of support there.