I would suggest you get a few centuries under your belt before attempting anything really long. As you go further you need to think more about food but not massively. You’ll probably need to stop from something savoury and filling rather than just eat out of your pockets. Other than that it is a case of start slow and stay steady and keep fueling. The only exception to this is if you throw in some steep hills. If you can spin you can go forever but as soon as you go into the red you will suffer and it will affect your ability to ride long distance. At that point you need to start gaining some more serious fitness.
200km (~125 miles) is a nice target, then 300km, then 200 miles then it is just a case of finding motivation.
Did my first 200 mile ride last year with about 3500m climbing (flat as we could really round here). Got in plenty of rides of 150miles, some with over 4000m of climbing.
LAst weekend we hit our next big target, riding from London to Newcastle in 21 hours (18 hours pedalling), a touch over 300miles.
Som of these took a lot of prep, working out where to ride, where to stop and making sure we weren’t caught out by light and bad weather.
Mostly a mental game though. It rained for the first 120miles of our 300 miler. But a good group meant we kept motivated even if it was thoroughly grim at times.