Good luck on your ride. I think its mostly mental game. If you rode 120km before during the week (combined rides) you should be able to complete it physically. Not sure what's your average ride length during the week but for me its around 40-45km. What I do when I start long ride I mentally say to myself this 40km is a warmup. I can visualize that distance, effort and time because I do it often. After a warmup I actually start my ride. This takes me to first half of a ride with little effort. The only difference is to pace yourself. Prioritize comfort over speed. If you can seat in the saddle for 5 hours you will make it. In the beginning of the ride go slower than normal (during your short rides), because you will likely run out of gas. Fuel yourself, eat something at least once an hour. Alternate food types if you can. For instance I would it a bar after first 1h then 30 minutes later gel pack then some figs and then go back to another bar. If you have sugar mix in bottle alternate that with normal water between solid foods. I'm usually keeping most of the sugar mix for last hours of ride because its harder to eat solids. If you fell like you have in you to finish strong later during the ride, go for it. The longest ride I made using this strategy was 260km. The biggest problem for me during long rides is to stay comfortable.
I think a lot would depend on what's your style, where are you going to be cycling? There are some general categories like commuter, mountain, road, recently more popular gravel which now days are more like mountain bikes from 90's. Most important is to start riding wherever you are, whenever you have time. Pick something you can afford and won't break your heart if you drop it (like 10K super bike).
Most of the time if I ride it's in the morning. For me its the easiest and most reliable way to plan and execute before day starts. There is also this feeling of accomplishment after the ride and it feels great the rest of the day.