I'm in the same boat. Hadn't ridden since I was a kid, brought an EZ Sport last summer, rode for a while then put it away. Got back on it about 3 months ago and know I'm hooked.
I'm mid 50's, 5'8", 210 and out of shape (sorry ladies!). I started by riding around the block a few times a couple of times a week. I'm now up to 15 miles twice a week on Saturday and Sunday. Can't ride more as I live in AZ, day time temps this time of year average 110, night time mid 90's. Once the heat breaks in late September I'll add at least one night to begin with.
Set weekly goals, each one doesn't have to be to far past the last, maybe just a block or two, pretty soon you'll have the distance your looking for. One thing I can tell you it's a great feeling riding past a former goal you may have struggled to get to a week or two ago. Now your passing it as if it means nothing and not even breathing hard.
Good luck, enjoy yourself!