What kind of clipless pedals? How much float do they give you.
You can't really think that you could ride like you did 14 years ago, do you?
Your body changes all the time. It will change as you get in better riding shape.
I think you should spend the cash and get fit/looked at on the bike, at least that will give you a starting point.
I also agree that your overall lack of fitness played a major part in this.
I end to think that you may have gotten tired and tried to push to great of gear to get home.
Not meaning to, just happens to most of us a time or two.
I would say not to try to bite off so much next time.
The difference between riding a "road bike" and riding upright can be a huge one.
Both in how you sit on a bike and how much you use your core to ride it.
Don't give up on it, it just takes time.