My recommendation for a first-timer is a loop route-- starting and ending at your house-- of ~34 miles. For your first imperial century, I recommend keeping the elevation gain to a relative minimum-- say ~4,000ft of total gain, around 1,200-1,400ft per loop at the most. Ride it three times. If you decide "today is not the day," and bail after two loops, you've still ridden a metric. The first two loops will be easy. The last twenty miles (at least for me) is when I start asking myself unanswerable questions. Even now, after having done my fair share of them.
There's nothing magic about that hundred number-- my first time out I didn't do a loop, took some wrong turns, and ended up with 109 miles-- it's more about the mental aspect of it than the physical. Pretty much anyone could get on a bike and ride a hundred miles if given no set time limit. You could do a barely-putting-in-effort pace around a park for 12 hours and knock one out if you could handle the sheer monotony of it.
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