I posted this somewhere before, but I think it's worth another look, even though it's more about zone 1 than zone 2.

Case study. I assume the guy's focus was time-trialing.

Very short version: former Norwegian pro soccer player, Knut Anders Fostervold, took up cycling and, after training very intensely for a couple years, he did "well". After ~2.5 years of doing lots of 4min zone 5 intervals, he drastically altered his training by cutting his zone 5 stuff heaps, increasing his z4, and more than doubling his time on the bike, almost all of which was done by quadrupling his time in zone 1.

After 18 weeks of his new program, he improved markedly, and eventually got himself on the podium of the Norwegian road TT national championships, beaten only by 2 pros, one being Thor Hushovd.

Summary (sightly longer version): before his training program was modified, he was doing 45mins/week of 4min intervals in zone 5, no zone 4 stuff, some zone 3 (~30min/week), and a few hours a week in both zones 1 and 2. His new program cut the zone 5 stuff dramatically (to 5min/week), increased zone 4 from nothing to 40min/week, increased zone 3 a little, cut zone 2 work to very little (to 1hour/week), and virtually quadrupled his zone 1 work from ~4hrs/week to ~15hrs/week!!

The revised program resulted in an increase of his Vo2 by 11%, and his threshold power increased 14%!!

It should be mentioned that the guy has a naturally high VO2, and was possibly frying himself for the previous 2.5 years with all the Zone 5 intervals, right? Also, perhaps he simply needed a change and/or a 'rest'. Perhaps the intense stuff built a good foundation.

Longer version (the article):
-----------------------------------------------

Case study #1
From Soccer Pro to Elite Cyclist

Intervals, Thresholds, and Long Slow Distance: the Role of Intensity and Duration in Endurance Training

"Knut Anders Fostervold was a professional soccer player in the Norwegian elite league from 1994 to 2002. A knee injury ended his soccer career at age 30 and he decided to switch to cycling. Knut had very high natural endurance capacity and had run 5 km in 17:24 at age 12.

After 15 y of soccer training at the elite level, he adopted a highly intensive training regime for cycling that was focused on training just under or at his lactate threshold and near VO2max; for example, 2-3 weekly training sessions of 4-5 × 4 min at 95 %VO2max. Weekly training volume did not exceed 10 h.

After 2.5 years of this high-intensity, low-volume training, Fostervold initiated cooperation with the Norwegian Olympic Center, and his training program was radically reorganized. Weekly training volume was doubled from 8-10 h to 18-20. Training volume in Zone 2 was reduced dramatically and replaced with a larger volume of training in Zone 1. Training in Zone 5 was replaced with Zones 3 and 4, such that total training volume at intensities at or above lactate threshold was roughly doubled without overstressing the athlete.

The typical effective duration of interval sessions increased from ~20 min to ~ 60 min (for example 8 × 8 min at 85-90 %HRmax with 2-min recoveries). The intensity zones were initially based on heart rate but later adjusted relative to lactate and power output measurements made in the field. Table 7 shows the training intensity distribution and volume loading for the athlete during the season before and after the change in training to a high-volume program. Table 8 shows the outcome.

Table 7. Comparison of weekly training intensity distribution and total volume in 2004 season and 2005 season – Case 1.
Intensity zone

.Zone...................old program.............new prog
(%HRmax)..............hours:min..............hours:min
5 (95-100 %)............45m (8.5 %)..........0:05m (0.5 % of week)
4 (90-95 )................0..........................0:40m (4.0 %)
3 (85-90 %).............0:30m (5.5 %).......1h:00m (5.5 %)
2 (75-85 %)............3h:05m (36 %).......1h:00 (5.5 %)
1 (55-75 %)............4h:20m (50 %)......15h:20m (85 %)
Weekly totals...........8hr40m................18h:05m

Table 8. Physiological testing before and after training reorganization – Case 1.

...................Pre......after 8wks......18wks......improvement
VO2max.........81............90................88...............11 %
VO2max........6.8............7.3..............7.3................7 %
LT power......375w.........420w............440w............14 %
W/kg-1:........4.5...........5.2................5.2..............15 %

The athlete responded well to the training load amplification and reorganization. During the 2005 season, after 2.5 y performing a low-volume, high-intensity program, a season training with higher volume and lower average intensity resulted in marked physiological and performance improvement. Although the athlete’s training de-emphasized both training near his lactate threshold intensity and training at near VO2max, both of these physiological anchors improved markedly.

Fostervold won a bronze medal in the Norwegian national time-trial championships, seconds behind former world under-23 time trial champions and Tour de France stage winners Thor Hushovd and Kurt Asle Arvesen. His failure to perform even better, given his exceptionally high VO2max, was attributed to poorer cycling efficiency and aerodynamics and a lower fractional utilization at lactate threshold compared to the best professionals with many years of specific training. In 2006 and 2007 he represented Norway in the world championship time trial. His absolute VO2max in 2005 was equal to the highest ever measured in a Norwegian athlete.