I am trying to keep to a low fat diet so I eat chicken,turkey,fish and plenty of fruit and veg.
As far as my workout goes I train twice a week Monday and Friday, last week was my first week, mostly on a crosstrainer and cycle 20 mins on each and treadmill 10 mins.
As I have osteoarthritis in my knees I haven't done any high impact weight training on my lower body, only circuit training on my upper body so 2x15 reps x5 kilos on three weight machines.
Any info for strenthening my legs and back would by helpful, i'm not sure what to do.
Thanks.

twice a week is not enough mate !

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As I have osteoarthritis in my knees

First of all I have a damaged knee and am awaiting X-ray results so I am in a similar positon.
1. The treadmill is doing neither you nor your knees any good as the time you are spending is not enough to do any good but will cause you more problems knee-wise. The cycle is better as there is no weight put on the knee joints, unlike cross-trainer or treadmills.
2. You can do weight training without putting pressure on your knees, but twice a week is not going to get you anywhere. Make it 4 times a week and the WOs about 1 hour, or until failure, in length.
3. Your diet need to be high protein but with sufficient carbs to fuel your WOs. I suggest you paste a copy of your diet here for the guys to see and advise on.
4. How much water are you taking each day?
5 What current exercises are you doing to strengthen your back?