If you can do thirty miles in a day on a reasonably regular basis, you can do 100 miles as a one off. If you struggle to do ten miles in a day, then you won't.
One hundred miles in a day is not that tough if you're reasonably fit; but you do need to be reasonably fit.
Won't help unless you are on a hand cycle. The only way to get cycle fit is to cycle.
If you can't face going out on the bike right now, swimming is better than sitting on your [censored] watching the gogglebox. Any form of cardio-vascular exercise is better than nothing.
But yes, if you want to do a 100 mile ride then you can't train for it in a pool.