Today I read the programme and executed it correctly. The alternate fast and slow lengths felt more comfortable although I know that I was holding back at the beginning as I knew I'd 40 lengths to do. It is important in interval training to work hard, but it is essential to be sensible at the beginning so you can do hard work near the end. Early burn-out means that the intensity level won't be sustained as long as intended.
Anyway, the result of the pacing was that all of the fast lengths were better than 2:00min/100m, with four at 1:50min/100m or better. Overall the numbers are pretty similar to last week except that stroke count might be a touch higher.