All I can say is, don't feel bad about doing 6 to 8 miles a time, if that's all you can do. Remember, you're just starting out. When I first started, 3 miles was about enough for me, but then I was on a heavy, ill-fitted mountain bike. But I started pushing myself to go further, to climb more hills, and stretch my endurance. I got to where I would ride 10 miles every morning (which is still my usual morning workout), and then riding longer distances on the road. I started setting goals, first was a half century (50 miles), then a metric century (100 km/63 miles), then a full century (100 miles) which I've done twice now. Maybe some day ride across my state, and who knows? Maybe eventually ride across the country. I also set goals with climbing local hills. There are not many hills around here I haven't conquered.

But the point is that, as a beginner, expect to have to build up your endurance. Set small goals. First, try a 10 mile ride. Then push yourself to do a 20 mile ride. Then maybe 30. Before you know it, you'll be riding half centuries often and climbing small hills at 15 MPH in high gear.