Like most fitness questions', the "how many miles?" answer is one of diminishing returns. So you'll get the biggest benefit from the first mile. Mile 2, not so much.
That's not to say that you only need to ride one mile a day to prepare for a 100 mile ride. 60-75 miles in a day would be a better answer, because you'll want to accumulate the benefits or more miles to enable you to ride that long distance without (as many) endurance and fatigue problems. But perhaps you have to ride two miles, #74 and #75, towards the end of your long training ride to accrue the same benefit as your first mile.