Training programs for new cyclist

I just picked up cycling because i am a senior running xc but have gotten injured. In my past 2 weeks off i began to use the stationary bike in the gym and usually do the hill intervals which is 125 watts hard and 75 watts easy. This has become pretty easy and have gone 50 minutes hard getting 12.6 miles in which is 3:58 per mile (im not sure if cyclists go by minute miles). On the weekends i also take a spin class in my local gym because my dad teaches them. The most mileage ive gotten on the stationary bike has been 18.3 miles at a fairly intense pace. I pretty much go hard every day because i really want to stay in shape and can handle the workouts since there is no pounding that would worsen my injury. One of my problems is i cant get really winded on the bike unless i put the resistance really high, and even then ill feel it in my quads more then my breathing, even though i can already feel a difference from the 2 weeks of biking and my quads have gotten stronger. I was just wondering if any of you guys can help me out with a training plan that would be pretty challenging and benifit me cardiovascularly. Also, when im on the stationary bike i average about 115 rpms even on the hills, and near the end on flats i can be close to 140 without to much strain. Thanks in advance for your help.