Those look good. I'd ride 3 or 4 days a week with an increasingly longer ride one of those days. If you can ride 50- 55 miles without issues, 100k won't be too difficult.

Have fun!

I think that the main reason why there's not a real prescriptive "do this and all will be well" for "century rides" is quite simple really. Riding 100km isn't really all that difficult for a reasonably healthy person with a half-decent bike. Unlike running, where you've lots of "getting acclimated to the impacts" for your knees, ankles, hips etc - which is what the whole C25k program is based on, gradually upping the distance first by walking, then walk/jog, then jog walk, then jog/run - riding a bike is quite simple. If you're fit enough to do a normal manual job for 8 hours a day, and certainly if you're fit enough to run (or run/walk/stumble) a marathon, then the only real issue you're going to have in completing a 4-5 hour bike ride is "contact point issues".

Get a good bike fit - have your bike setup properly, and buy a couple of good pairs of cycling shorts - spend some money on these, believe me it'll be worth it - Go for good Brands, Castelli, Assos, Sportful - remember it's the quality of that padding that'll get you through the 3rd to 5th hour in the saddle.

But yeah - the linked sportive plans WILL work - they're mainly focussed on people doing specific "Challenge Centuries" or Sportives. Which in turn tend to be hillier than a typical "just riding along" ride unless you happen to live in a mountainous area.

Personally, I'd take it quite simply

Monday - rest day
Tuesday - 1 hour after work, Focus on pedalling technique and keeping a fast cadence on flatish roads.
Wednesday - 1 hour after Work, Find some hills and ride up and down them. Try and keep cadence high
Thursday - as tuesday
Friday - Rest Day
Saturday - 1.5-2 hours, just riding, nothing silly, just enjoying yourself.
Sunday - 2-3 hours, again, leisure riding, maybe a cafe stop mid ride.

repeat this for 6 weeks, adding 10 minutes to every hour quoted. That'll mean that on sunday, you'll be doing between 4 and 6 hours in the saddle, which MAY even mean that you've already hit a metric century in training. But even at 4 hours, you'll be doing at least 80km, and you'll realise that its not much of a stretch to hit the 100km.

take the 7th week off (or just ride the distances from week 1 - it's called tapering ), and ride the century on the 8th week.

simples

I don't think a bike fit is going to make you much more efficient, but it should make you much more comfortable! and that's pretty important. Also, a poor fit can result in stress injuries, body part numbness, etc.

The big thing with a 100km... manage yourself... drink regularly, eat regularly... don't wait until you're thirsty or hungry, it'll be too late. There is no more helpless feeling than running out of energy (bonking (runners say hitting the wall))... most everyone that rides seriously has done it, and it is completely avoidable. Try to consume 250 calories an hour while you're riding (Clif bars work for this quite well). I can go 50 or 60km without food without a problem, but when I'm doing 100km, I have to start eating by 30km or else I WILL run out of juice (did it twice last year, NEVER again... )

Everyone here has good plans. A few simple truths: train fast to ride fast. Ride long to ride long. Experiment in training with clothes, nutrition etc. (no changes on event day). It's exciting when you finally get started in your event but make it a point not to go out too fast. A lot of people (me) get up early to drive to an event and tend to forget water bottles, helmet, shoes, garmin, phone, even bike or front wheel....a checklist or post it might help.