Returning to bicycling after a long hiatus
Age is a big factor - if you are 20, you probably can get away with going straight into it, if your over 50 (like me), the body is less willing than the mind, much better to take it easy. Some people bounce back fast, others take a bit longer, so there is no way to say what you should do.
Main risk is going too hard, too soon and suffering an injury. Start with short and slow rides, listen to your body, and build up to your 20 miles. Over doing it and injuring yourself means you time to get back to normal capability is significantly increased and inconvenient.
Keeping in mind the age thing, what I would do is something like 5 miles slow, adding 10%-20 per week until your up to 20miles. Mix it up with some short faster rides, increasing speed by 10%-20% effort (not actual speed) per week.
You get do not get fit exercising, you get fit recovering from exercise.