If the gears work, the chain doesn't skip on the small cog, the bike fits, the brakes work, there are no cracks or big dents, and the wheels turn nicely, and the rest of it looks safe to ride, then sure, get it (if the price is right) and welcome to the sport / pastime / mode-of-transportation.
If you immediately spot several things that need replacing; tires, brake pads, rear cassette (chain skips), those can add up fast, and you should budget accordingly.
Any bike that fits you well, that has gearing that will match your type of terrain and fitness level, and that is in good working order will suffice for 25km - 50km rides. The limiting factor will be your body's level of adaptation to spending hours in the saddle. To that end, start with 10k rides and work your way up until you reach your distance goal.