Forgot to mention, concerning my problems doing a 60 second interval. I can do it, but really have to force myself - and then I find it hard to do another, at least of that length.

But I can just keep riding normally after pushing that hard without problem. It isn't like I'm gasping for breath or something.

I think there could be two problems. One may simply be mental - my brain just says to back off, but maybe I really don't need to.

The other would be the simple fact that I have a heck of a lot of miles over a number of months where I almost never really pushed hard. I was content to be doing my first 11 miles at 15 mph or better. So, perhaps my muscles need retraining - they can go a long way, but not push with enough power to get the speed. When I max out on an interval, the muscles really don't like it. Hopefully doing some of this will alter that.