In reply to IainRUK:
> (In reply to David Barratt) If you've dodgy knees I'd refrain from adding weight..
When I was training for the mountains with a dodgy knee, I used ankle weights on a treadmill (at max. inclination) and/or step machine. Found it was fairly effective and didn't stress the knee. Cross trainer also good but, as mentioned above, you have to crank up the resistance and push hard to get a reasonable workout.