50km for someone who has no experience cycling long distance
I can just imagine myself back then when I bought my old vintage bike and rode it 103km to my hometown without proper training or gear. There's a lot of hills along the way but I manage to reach 60km pedaling my bike before my legs felt tired and started walking on climbs but still pedaling on flats and down hills. I have no helmet and cycling shorts back then, so sun burn and butt soreness was really an experience I don't want to happen again. I have learn a lot from that experience. Now I ride 40-50km every weekend for practice to prepare myself for another epic and longer ride.
From that experience, I recommend you to consider the following.

For you:


Health - If you are healthy enough without any injuries or any health issues which can be triggered by fatigue, then 50km is good.
Terrain - Terrain is the most important part when you go on a long bike journey. This will be the defining factor regardless of distance. Just imagine the amount of effort you will exert when you go on a 50km hill climb vs a 50km flats. The more effort/power you apply, the less gas and distance you can finish. I suggest you read a map of your desired route and look for how high or low you'll be climbing. If you're sure it's all flats, then 50km is no worries for beginners. If there are a lot of climbs, then you might shorten your desired distance first for maybe 15-20km.
Speed - Same law applies with terrain. More effort/power the less gas &amp; mileage you can accomplished. So make sure you're only pedaling at your comfortable/easy cadence. You want to tour then you have to take it slow and enjoy the journey.
Padded Shorts/Pants - Trust me, butt soreness is never a joke. This is a must.
Protective gear - helmet, gloves, reflectors or lights (if you're riding at night), UV protected clothing against the heat of the sun or jacket if it's cold.
Food - Bring plenty of carb rich food like bars or gels.


For your bike:


Wheels - Make sure your wheels are still in good condition and bring extra tires, pump, tubes if you're in a clincher wheels.
Water - Bring enough water. 2 bottles will do.
Toolkit - For any repairs along your journey.
Money - If you want to buy stuff along or worse your bike broke and you can no longer ride it safely, then a payment to a nearby repair shop or a ride home.


Is there anything I missed? Feel free to edit this answer or comment down below.