There's a book titled "the first 20 minutes" that makes the case for high intensity interval training. Worth reading. IMHO, any rounded cyclist will want to ride long distances with friends so that's one thing to work on. Training to allow yourself to ride 60, 80, 100 or more miles is a thing in itself. You can't do that full tilt boogie, but full tilt boogie is where you develop serious cardio. If your 60/80mi or whatever long distance rides include hills, you can do interval training on them or even on flats, but anaerobic training and high level, hard aerobic training is best done on hard, fast, shorter rides so I'm with those that suggest both the 35mi slower rides and faster 20mi rides are equally important. I would note, though, that the shorter, harder ride is probably doing more for your cardio than the longer ride. You could continue to shortern that short ride so it's shorter and even faster and harder and that would continue to increase the cardio benefit. The hard fast riding is what helps build the cardio that allows the long rides to happen but the entire body and muscle training needed for long rides has to occur as well. That's why most serious cyclists find themselves doing fun rides of long distance at a pace that can be maintained and do one hour rides about as hard as you can. That's how it is for me, anyway and I'm a older non-racer that just enjoys riding as well and as quickly as I can. My one hour rides all include a half dozen sprints. I do actual high intensity intervals in spinning classes where I can push till I'm dripping with sweat and near collapse. THAT level of stress on the body helps the body to react to build endurance and it's hard to do that when you're actually riding. That's probably the biggest "sell" for indoor training.