Cycling is ideal for the cardiovascular system. You can train for hours without overloading yourself orthopaedically.
Tip 1: Check your bike at your dealer as well as the material. If you start cycling again after a longer break, e.g. after winter, you should check your equipment for fitness to ride. Especially in road traffic this is vital.
Tip 2: The frequency makes the difference. I'd rather go shorter, but more regularly. E.g. three short one-hour after-work sessions are more effective than five hour rides once a week.
Tip 3: No monotony in training. Try to vary a little in each workout. Adjust duration, intensity, route profile etc. again and again. This enhances the training effect.
But despite all above-mentioned tips, the most important thing is to enjoy it and have fun!