hiya sis! LOL... yes, by all means have fun...

the trick to riding is... well, riding! Really it's all about just getting miles in your legs... the more often you ride, and the further you ride, the more miles and faster speed you'll be able to go. Stopping is fine when you need to... just keep in mind that the longer you stop the longer it may take your muscles to warm back up... this probably means nothing to you right now... but it will as you continue riding!

And yes, if you can ride 21 miles, you can certainly work your way up to a metric by March... welcome to your newest addiction!