Speed and distance for a beginner I'm 26 and haven't ridden a bike since I was 15.
I decided to start again and purchased a nakamura crossland, last Saturday, to get back into action.
Last Sunday, I did 17km in 50 minutes on a mostly flat road. I didn't want to start too hard, afraid to hurt my legs. In the end the only thing that hurt was my butt, I probably need a better saddle.
I was wondering what distance and speed I should aim for as a beginner, in order to improve my stamina.

P.S. I decided to purchase a bike to lose weight and enjoy the countryside. Maybe it's relevant for choosing the right distance.
A few simple tips:


Whether it's cycling, running or any other physical activity the more gradually you build up your mileage, the better off you'll be in the long run. A good rule of thumb is don't increase your time or mileage by more than 10% a week. 
Carrying on a conversation with someone should be difficult but not impossible. If you can't carry on a conversation you're probably pushing too hard. 
Consistency trumps volume. 3-4 short rides a week will do more for your overall fitness than 1-2 longer riders each week. 
We can not get grace from gadgets. It's easy to get sucked into buying ever newer, shinier and lighter parts. However, if you think these things make a big difference you're fooling yourself. Keep in mind when you're pedaling up a hill you're not just pedaling a 14 or 20 or 25 pound bike. You're also carrying your weight and the weight of anything you have with you (such as water bottles). When you look at a combined bike and rider weight of 170 pounds or more suddenly saving 30 or 50 grams is a minuscule difference. I've wrenched for several professional racers and trust me, most of them couldn't care less about their bikes. 
Do invest in cycling shoes and clothes if you're going to do longer rides. Cycling shorts - whether the baggy style or the lycra type - are designed to reduce friction. Cycling shoes with clipless pedals are more efficient and also safer (once you get used them - which takes some practice). 
Don't forget to drink. It's easier to get dehydrated while riding than in most other activities. You're moving faster so the sweat evaporates faster and that fools you into thinking you're not sweating that much. 
Enjoy riding. I've seen people who bail on a ride because they couldn't get Strava to work. That's insane. Ride because you want. You're not a pro, your paycheck doesn't depend on how fast and how often you ride.