I will preface by saying that you should know the difference between pain and soreness. Soreness is good – you are working your muscles. Pain is bad – can lead to real injury.
So I am heavy but have been riding for 4 years pretty consistently. I have lost no weight but have been in good shape at times. I had been riding to work last year. 50-100 times 13 mile round trip. Now I didn’t lose weight because I was so hungry that I would eat a meal at 3am just to be able to get back to sleep. But I was in good shape.
Regardless of how much skill you have being in shape makes riding more fun. When you get to the top of the hill and feel good, the ride down is the pay off. If you feel bad, the ride down is just survival.
Not in great shape now due to some injuries and no reason to ride to work. On a recent ride I was looking forward to a series of seven tabletop jumps to work on. They were on the last trail of the day. I cramped immediately before them. Had I been in shape, I would have done them twice. They were the pay off for a 15 mile ride.