Ditto what everyone else said, and a question: are you spinning, I mean as much as possible, rather than trying to push hard gears? I only ask because you said your muscles get tired. If not, then try spinning lower gears and see how that feels. If you're already doing that, then, yes, just keep adding a mile or so whenever you ride and your distance will accumulate before you know it.

I think you're absolutely right about keeping it fun and do-able, so you'll continue to enjoy your rides and not injure yourself. Just keep at it--you're doing fine!

Try throwing in something fun--like Mimi's Dairy Queen ride idea--or even wacky once in a while. This morning my neighbor and I rode to the local Waffle House for breakfast--we wore our girly skorts and had a great time. Only 12 flat miles total, but fun fun fun!