People often worry about what training they need to do in order to be ready for an audax. You've decided to enter a 200km or longer ride, and this is surely going to require an extensive training programme and a tapering phase, right? Well, everyone will be different with their ambitions and capabilities, but my personal view here is that preparation is what really matters. Any audax attempt should be based on an accumulation of experiences in previous rides, so let's examine what aspects should be considered for you to be "ride ready".
The lower speed limit (15kmh) for an audax is quite generous, so riding one is very much about controlling your efforts so that you're able to complete it without too much physical stress. This doesn't however mean that you can expect to do this without a steady stream of riding under your belt. I think that it's better to do lots of regular short rides (almost every day) to tune your body and mind into the necessary motions, combined with occasional longer rides - but again, I'm only talking about club rides here, 80 to 100 km every week or two. The goal is to reach a "comfort plateau" where you feel good about steady riding for many hours at a time.
So in summary, I don't think about "training" or "tapering" (where you reduce your workload in the run-up to an event). I just make sure that I'm healthy and that the legs & body are comfortable with that prolonged time in the saddle. The audax itself just forms part of my cycling schedule.
It's really important to study the route, so that you know where to find decent stops (for refreshments and potentially a rest). It's also good to highlight any tricky bits on the ride, especially any hills. I strongly recommend creating a route card that attaches to the handlebar, listing up to 10 of these useful waypoints and their distance along the route. This way you can mentally prepare for the potential "low points" which could prove your undoing if you don't see them coming.
Leisure cycling should be a natural antidote to putting on weight, but this doesn't mean that you can just subsist on junk food! The lure of cafes and the need for calories means that you will probably not eat terribly healthily whilst on the bike, but day to day it's important to have a balanced diet with lots of fruit & veg (of course) plus high fibre foods too.