Training and preparing for L’Eroica is not provided in some sort of online program or printed guide book. Perhaps the best advice I’ve read so far is not specific to L’Eroica, but to any demanding bike ride or race or physical activity. It is summed up handily in Rule 5 from the Velominati site.
Taking that as my guide, I am making a point now, just five weeks and a few days from the opening bell, to always go an extra few K on every ride. If I have a known route that connects to another, I combine them. If I have an existing route that can be extended, I do that. When the alarm goes off at 5:45 and I know I could use some more sleep, I get up and put the kit on.
This has been working well, but I wonder if it’s enough? I have yet to ride anything approaching a 205K distance. I know that I may not actually do so because training doesn’t necessarily require it. One significant element that I have yet to do much of is riding on the pebbles. The following link will show a route that I put together from a couple of others. It does include a 5K climb on gravel, though the surface is pretty forgiving.
L'Eroica Dreams 
You should stencil Rule 5 on your bike shorts.