Well, 10 miles is a good start. That was my normal ride length when I started, and it was hard at first (I'd come home with jelly legs). Still, test ride some other bikes, because there might be something better for you, one that will make you say ahhhhh when you hop on.
100 miles is piddly around this forum, but to compare is to despair! Do what works for you, and that might be half as much, or twice as much. When you get it right, you'll know it because you won't have any repetitive injuries, you'll have energy and feel great, and still time for other things.

