Never had to "break in" a Brooks.
Personally I believe they work mostly by forcing you to more quickly adapt to sitting (i.e. putting more weight) on your sit bones. The sooner your body adapts to doing this the sooner you can do silly distances on the saddle and not worry.
Many other saddles have padding on top and the body uses this, and various muscles in the backside, to keep pressure off the sit bones for as long as possible. This is why a saddle that may be comfy for daily commutes can rip you to pieces on a longer ride.
It's also why hundreds of hours on a normal saddle, one hour at a time, does little to condition your sit bones, as your body has been avoiding doing so on those short rides.
With a Brooks you have little choice from the start, the lack of padding means the muscles soon give out and you start conditioning your sit bones much earlier into a ride and, more importantly, on pretty much each and every ride you do - no matter how short it is.
However, it's not all that simple. The less "Brooks shaped" your arse is the longer it will take to for things to become comfortable, this is what some people erroneously describe as "breaking in" the saddle. A badly fitting saddle is going to hurt regardless of whether it is padded or leather. As others have said, it's not the saddle that is being broken in, it's sometimes you.
(The type of saddle is specific to this - either because of design or the different grades of leather used - and it's probably specific to individual saddles too. I had a B17 that I just couldn't ever get on with and eventually sold it, but looking back it's probably because I was doing most of my riding on a "normal" saddle and only using the Brooks fitted bike for infrequent long rides.)
FWIW I had my fixed Audax bike (including the Brooks Swallow that was on it) stolen a few months before PBP in 2011. I got a replacement bike through an insurance claim and collected it, with a brand new Brooks Swallow on it, the week before PBP. I got to ride the bike twice (half an hour each time) before I handed it to someone to take to France for me in their van. Once out in Paris I did 4 trips between StQeY and the hotel (10km each way) before starting PBP. So I ended up riding 1230km in under 4 days on a Brooks saddle that had been ridden for a whole ~60km beforehand and I didn't have a problem at all. The saddle did not need to be broken in, my sit bones were conditioned by plenty of riding on the previous Brooks Swallow before.
Also; Proofide is for replacing the natural oils that are leached out of the saddle by it getting wet. It also offers a small amount of water resistance (which is why it's suggested to apply a small amount prior to first use). Too much Proofide or too frequent applications will just lead to the leather stretching as it becomes too supple - it's a quick way to ruin an expensive saddle.