The most important thing is to develop your own style. How far do you want to cycle? How fast or slow can you go? You won’t know until you get out there and get your legs moving.
For touring and traffic-free cycling your bike should be big and sturdy enough for beaten paths but light and smooth enough to make pedalling fun. You get what you pay for. I had a heavy cheap bike for years and my current bike is mid-low range. Ideally you need enough gears to avoid grinding (pedal at a low comfortable gear). For a while I had my seat (a fat gel) too high (too low is more common for beginners). Basically you need your leg to be slightly bent when the pedal is down. You also need your hips and seat (bum) to stay level when you are cycling. And stay seated for goodness sake! Touring for most of the day is nothing like spinning in the gym so you need to pace yourself.
I don’t always get it right but those last pushes on challenging days often make the most memorable moments. You don’t necessarily want to push yourself too far. I heard of a man who cycled the Coast to Coast (140 miles) in a day then knocked himself out by falling over in the shower. Eating and drinking on the ride is important and often a great joy. I don’t get scientific about it but getting my nutrition right tests the limits of my common-sense.
Mileage can be anything you want. Moderately mobile beginners will be able to manage 25 miles to 50 miles a day – in kilometres this will be impressively more! Remember, you can take all day and you are on your own journey. For me slow living is good living. A ride can feel like a whole weekend away and a tour makes me feel like I have gone back in time and turned back the clock.
Oh yeah. I forgot to tell you that I am still only 21!