You sound very much like me when I first started riding. I too had just lost a bunch of weight (congrats, by the way!) and was in good cardio shape.

My very first ride outside my neighborhood was a weekend ride of 15 miles of rolling terrain. I took my husband with me and he HATED it. He almost put his bike in the car to return it to the LBS that afternoon! I, on the other hand, loved it and was ready to go right out and ride again.

I rode another 15 miler with a few guys at work a couple of days later (they took it easy on me) and then a 10 miler later that same week (man, did my sit bones HURT!). I did my first 'long' ride of 36 miles that second weekend. That ride was an organized training ride for our MS150, so there were tons of other cyclists and rest stops and everything...that made the distance a lot easier to handle. I'm sure I wouldn't have gone nearly as far that quickly had there not been other riders out there (and rest stops with porta-potties!).

Oh, by the way, my husband didn't give it up and now rides regularly - 4 years later.

My issue wasn't my strength or my cardio capacity - it was getting used to the position on the bike and getting my butt accustomed to the saddle. I worked my way up slowly after that first week so that I could ride in the MS150 a few months later. Having been in really good physical shape before I started riding really, really helped me adjust quickly. In fact, I find it harder to get started after a few months off the bike now than I did when I first picked it up just because I was in such good shape then (and I'm not now!).

Welcome!