Hi Daz, good luck on your new drive. There's a fair bit of info on these threads now so hopefully you will find something useful.
In a nutshell you have to sort out a suitable training programme which meets your goals(fitness, weightloss, muscle building whatever) and also overhaul your diet to cut out the beer etc.
As you know Karate so well, maybe this can be your main activity followed by some gym work if you feel you need it.
Set yourself some achievable short term goals which help to your long term goal.
all the best
Paul.