My advice is this: patience and perseverance. I was a college football (QB) and baseball (SS) player. When I began cycling in my mid-30's I expected to do a few rides and be fit. It doesn't work that way with cycling. It takes a longer time to come to fitness. And, then there's the issue of technique. There's a lot to learn and it will take time. But, you will get there. Along the way try to learn something new every ride. It might be about your bike or shifting technique or climbing or the course. It's endless and that's what I like about it. And, most important, enjoy the ride.