is this LSD and will it do me any good?

How many hours are you riding a week now? Will you be adding these 100km rides to what you currently do, or substituting them for current rides?

E.g., if you're only riding 6 hours a week (all/most of them at your 31km/h pace) and you decide that 4 of those need to be done as LSD in the form of a single 100km ride, you will probably lose fitness.

If you're riding 12 hours a week (2 hours a day and one day off) and decide to take an extra day off but add 2 more hours to another day, you probably wouldn't notice much difference except maybe in your endurance if you haven't ever done long rides before.

If you're riding 10 hours a week and decide to add 2 more hours to one of your workouts as the 100km ride, thus riding 12 hours a week now, you should see some increase in fitness. But that will be from the added volume/training more than anything to do with how you ride that 100km.

To answer your question properly, it really depends on what kind of volume and intensity you're doing now.

Going from 80 minutes to 240 minutes will do you a lot of good.

But doing 120 miinute rather than 80 minute rides will do you a lot of good also.