I am not a fan of indoor pedaling. However, during the winter of 2016-17, I was doing intervals 3 times a week for a few months. The intervals definitely improve ones strength and aerobic threshold. I was in very good shape for riding come springtime, though I did have to work on the longer rides. The intervals also helped with keeping me interested and motivated. The downside for me, I developed some bad ride positioning, which led to being uncomfortable, achy and saddle sores. I had to mentally be aware of my positioning to correct that. I am pretty sure that a big cause of all of that was not having my bikes set up as close as can be to the same way for all of them.