I think that is the crux of Cambridge - for very little work you see huge rewards as the weight just falls off.
On WW and SW at 2lb per week 3 stones would have taken me 20 weeks-ish, five months.
I swim three or four times per week but that is about all I do in the way of exercise, so not over-exerting myself in any way. I have arthritis which badly affects my hands and I cannot hold onto gym equipment or weight machines, free weights etc, so that is all out. I have to do what I can which is to swim. Sometimes I cannot get into the pool but where I go to has an electric chair which puts me into the water when my hands won't hold the rails.
You have to find something you enjoy and something which you can do on a very regular basis.