Plateau

Good afternoon, its been 3 weeks since my last confession.
Recently i have noticed that I have reached a sort of plateau  ( i think that's what it is) , I'm working out as normal but I'm not really seeing any more improvements. I have been considering for a while of increasing the amount of cardio. I was going to start back running but the physio has advised me not to. So I have decided to take part in an on line challenge
http://www.menshealth.co.uk/building-muscle/get-big/sign-up-for-mens-health-total-body-challenge.
I was considering some sort of HIIT . Probably Tap out to do along with the kettlebells. so it would be Kettlebells in the morning and Tap out in the evening. The problem I am having at the moment is the dreaded middle age spread! you see like many men my age I have a slowing metabolism, and the belly is getting bigger and no matter what I do I just cant shift it. I have never once in all my 36 years been considered fat or overweight ( the complete opposite) and at the minute I am working out more than I ever did. In the past I worked as an outdoor instructor so I was active all the time, Hill walking, kayaking etc. So I was thinking that I'm not doing enough cardio to shift the excess fat that middle age spread is giving me. What do people think?
Will this be any good?
Should i just get bigger Kettlebells?

http://www.menshealth.co.uk/building-muscle/get-big/sign-up-for-mens-health-total-body-challenge
It might be a bit of craic. I have been doing my current workout since Christmas, so it might be time for a change.