From what I've read, I should have a good base before moving to interval. However, I joined an indoor class at bike shop, which had already been going a month or so this winter, and it was pretty much just dive right in. Tues. and Thurs. 20 minute warmup, hour and ten interval training, and cool-down. My only question is, what are the implications for my overall improvement? I'm doing a lot of zone 4 work. Is this good, bad , or other with virtually no base mileage? I can take the pain and strain, but how is this helping me?