You didn't start backwards! I know quite a few people at the gym who became hooked on road cycling because they met someone at spin who was a roadie.
I was a gym rat for years; taught aerobics in the 80s and 90s. My son started mtb, then road racing and then my husband started cycling. I was thrilled my husband finally was exercising, but it was hard to coordinate our schedules. Then one day he said, "I am going to get you out on the road." He put slicks on his old mtb and my first ride was about 5 miles. He had to PUSH me up a small hill with the back of his hand. It was as if I had never exercised a day in my life. This was in the fall. Then I went to spin with him during the winter and the next spring i started doing 10-20 mile rides. After that season, I got a road bike, and then a year later, another one.
You will be fine. It's a disease!