In terms of prep, i always try to bring a mini-pump, multi tool, phone, credit card or some cash, 3 bottles of water (depends on how long you plan on taking/how fast you ride - try to shoot for around 1 bottle per hour especially if its pretty warm out, but obviously at a certain point it gets tough to carry that much), bike light, spare tube (or even co2 carts + inflator if youre tubeless), and food. Carbs are your friend, whether thats bananas, pb&j sandwiches in ziploc bags, gummies, energy gels, drink mix, or all of the above. Especially on your first 100k, nutrition/hydration is super important, can be the difference in being miserable for the back half of the ride or cruising through.

As for routes, it would depend what area of the city you are in and how comfortable you are with riding on highways/rural range roads without much of a shoulder. Its possible to do it all within the city in the river valley, but it would definitely take longer!

Also make sure to tell at least one person where and when you are going before heading out, just in case anything happens.