Let me ask you this:

Are you looking for a bike purely for fitness, or do you have plans to get groceries with it, commute with it, go off-roading with it?

There's a massive range in bikes and what they can do... the joke that you always have "N + 1" bikes, isn't really a joke!

I've got a 90s steel mountain bike that is my hauler, bad weather, all-year-round, "it's only rain", "it's only mud", bike... but it's not practical for fitness/training.

I've got a folding bike that turns heads, but it's the slowest bike I've got, and I wouldn't dream of keeping it locked out front of a store for more than a second.

My last bike, however, is a gravel/road bike, and that thing is just for speed and distance. It's got no fenders, no racks, but can take me hundreds of KM at a time for fun.

I'd probably have 5 other bikes if space and money weren't a concern... so many bikes to do so many things!

Also, your goal to do a century in a few months as a runner, should be no problem! I went from ZERO riding experience only a few years ago to a metric century in under a month. A year later, 240+km in a single ride. This year, I hope to break 300km. Riding is fun! Enjoy, and yes, don't overtrain!

If you already use a Garmin device, the Garmin Cycling Coach (free version) works great, and can custom tailor your training to meet your goal, and will even adjust the schedule based on your progress and/or fitness levels.