I too have f**ked knees. They don't prevent me from running, but certainly squats and lunging.
I also pay £46 a month for my gym membership (actually it's a health club with access to all facilities, swimming pool, Jacuzzi, steam room, sauna, tennis courts, classes etc, but I only use the gym & pool).
The logic behind it being, at that price I'm loathed to not waste it - so even when I'm having a particularly busy day, I'll still make the effort to nip in there and try to get my money's worth.
Try to work your muscle groups in pairs too, i.e. 3 x 10 bicep curls - followed by 3 x 10 tricep dips. 3 x 10 leg raises, followed by 3 x 10 dorsal raises etc.
One sure-fire, 100%, fool-proof, guaranteed method of making sure you're making progress? About 1-2 days after your arms / chest / stomach etc will be in bits. By day 3-4 you're ready to go again.
Forget about weighing yourself. Ever. And forget about checking the mirror. It's like brushing your teeth, twice a day for two minutes does nothing right? Nothing you can see anyway. But over a period of time it makes all the difference.
One important point; you can't out-train a bad diet. One thing I found very hard in the early stages was eating what I needed and not what I wanted. Breakfast biscuits in the morning, tuna salad for lunch with a cereal bar, two pieces of fruit and a packet of Sunbites. And a sandwich in the evening. Every weeknight for the last 18 months and counting. Eat what you like at the weekend. Eventually you'll get to a stage where you just cannot eat anywhere near what you used to. Less carbs, more protein. Your body can't store protein either so save the steak / fish for about 30 mins post workout to help feed and repair the torn muscle fibres.
The other important point is for it not to feel like a punishment. Change one small thing at a time rather than go full-tilt. 3 months later, change something else. Then another 3 months later etc etc.
I used to miss having a cold beer in the summer weekday evenings, so I bought some Becks Blue. The difference being I didn't wake up at 3am for an "old man pi$$" and have a groggy head next morning. Three months later I switched it up again for orange juice. Another three months and Instead of 1,000-calorie fish and chips every Friday I now have corn chips (150 calories) with melted cheese (400 calories) & jalapeno (25 calories) over them. Or sausage sandwich with melted cheese (500 calories). Most recently I swapped the mayo for ultra light and Canderel for sugar.
Oh and if you want to lose about a stone in one weekend, get one of those microwave chicken burgers that has gone out of date and put lead around your toilet in preparation.
Long post, but hopefully some of you will find it useful...
You should write a series of posts like this and make them sticky. Lots of useful info here.