I think the posts have summed it up quite nicely. This should be an easy task. All you need to do is get your butt adjusted to sitting on the saddle for 4-5 hours in a row, and that will come with you just riding around.
I'd certainly try and ride 100km a few times, maybe even every weekend starting in a 4-5 weeks if you have the time. It's really not that hard to pedal nearly forever at a slow pace.
PS, whenever friends/family come to visit, I try and have them get on my guest bike and we go for a ride. Everybody thinks they will only go ~8-10km and be done, but we go 40km and they usually feel fine after. These are people that dont exercise at all, and havent ridden bikes in 20 years. You can probably do a lot more than you think.