My first ride was 6 miles. DH and I had just bought hybrids and took them out for a spin. When we got 3 miles from the house, I was in a panic because I didn't think I could make it back home so we turned around and went back. When we got home, we collapsed exhausted on the couch and thought we had really accomplished something--and we had. A month later we were riding 30 miles on our hybrids and bought road bikes; five months later we did the MS150 (75 miles 2 days in a row); a month after that we did our first century. You can progress pretty fast on the bike.
Just enjoy yourself and ride your bike. PamNY gave you some good advice. Don't forget to hydrate properly and eat--especially if the weather is hot and you are stretching out your miles. There are many threads on TE about hydration and nutrition.
Have fun!
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"We don't stop playing because we grow old; we grow old because we stop playing." George Bernard Shaw
Luna Eclipse/Selle Italia Lady
Surly Pacer/Terry Butterfly
Quintana Roo Cd01/Koobi Stratus
1981 Schwinn Le Tour Tourist
Jamis Coda Femme