Well done!
My 'Top-tip' for beginners ... take it easy, especially when you first set off, choose a low gear and gently spin along rather than give it the full Wiggo from the off. Warming-up gently is really important for those who maybe have not exercised for a long time or sport a notable 'aerobelly'

This is to allow your cardio-vascular system and fuel supplyto get going properly and you'll find that you'll go further with seemingly less effort if you do so...
Warm-down too, same thing, gently spin the last bit home.
Keep it up, as others have said, the miles build quickly.