I have never, ever done a block of L2/L3 work, so I have no opinion on the benefits of lots of volume of L2/L3. When I want to increase my endurance for an upcoming target ride, I maintain the same mix by level but simply increase my total volume as measured by TSS. Everybody is different in terms of their body's ability to handle increases of total volume, but I would be very cautious about increasing more than 10% week over week. Also note that if you increase hours on the bike by 10%, you will get more than a 10% increase in TSS due to the fact that you will probably keep your warmup/warmdown segments constant so most of the increase will be quality time. But, even if you increase only 10% per week, you can go from 10 hrs/wk to 13 hrs/wk in three weeks. That's actually a pretty big jump in 3 weeks.