Is it reasonable for a person who usually bikes short distances recreationally to sign up for a 20 mile bike for charity?

If it is for charity, why not? Unless you are very competitive, there is no rule to say that you must finish the 20 mile course. You can always stop half way if you feel that it is too much for you.



The most important thing is to have fun and feel good that you are doing something good.

Just make sure that the bicycle you will be cycling on is of the right fit for you. Even if you do not plan to ride to win, it is best that you learn how to prepare for it and to build up your stamina for the long distance ride.

You will find some of the information I shared in the following articles useful.

What are some good rules of thumb for starting out long-distance biking?
How can you ride a bicycle without getting tired?
Which is the best bicycle for long distance ride?
How many miles should I bicycle per day on a world tour?