Chunks replacing sections of my Minecraft Home

The Problem:
I began my minecraft world in one location, built a massive castle there, after some time I decided I would change scenery and build a house at the end of a massive rail line.

After I attempted to return home one time, I drowned in a wall. Confused, I noticed that instead of my usual rail network, a giant chunk of randomly generated land was blocking my path.

I found a different way to get to my castle and found other chunks that have been generated even though they should have loaded as I've already dug them out and built a castle around them.

What may have instigated this:
Setting my render distance higher than my rig could handle and travelling toward my castle appears to have caused it.

What would I like done:
To stop this happening in future and provide me with a way to reload old saves in the hope that minecraft stores multiple saves and doesn't constantly write over the one save file constantly, does it?

What are the photos I have linked?:

Some of them are the game in interrupting random chunk mode, others are in the mode where everything made sense, I really would like to return to that world again.