I agree with the Friel method and am currently embarking on a winter of L1/L2 training.

It involves a great deal of patience and a re-alignment of your expectations during this time. If your season was particularly hard, I find a long stretch of L1/L2 to be rather therapeutic, actually. Contrary to popular belief, there's a great deal of mental engagement involved in base training. You should be focusing on efficiency (i.e. pedal stroke), maintaining a high cadence, achieving balance on a bike... these things aren't easily developed over the course of a season. Not to mention a good dose of cross training, flexibility, core strengthening, weight loss, etc. is IMO critical to supporting a good season.

It takes YEARS to build a good aerobic base (ever heard of the expression "it takes 5 years to build a champion"? or how generally speaking, it's too late to become a world class competitor when a cyclist gets into the sport in their twenties?).

You'll get your gains by mixing in speedwork and going hard but what Friel et al. argues is you'll be leaving a good deal of potential on the table if you dip into >L2 too often