As Tom said diet, stop drinking and smoking.
We have a fully stocked free gym at work too, I'd been bugged by my workmate to go so I thought I'd give it a go and while he admits he's no expert the routine we do that fits into our lunch break has had an effect.
Now while this isn't an ideal situation I've seen a benefit and that in itself motivates me in other ways especially this time of year when the hibernation sets in.
Good luck with it, it's something I wish I'd turned to sooner.
Definitely be an interesting thread look forward to reading @Middle Class Caveman post.