FWIW, I have just completed a review of my 2006 training rides through mid-October (181 ride files and ~340 hours) from the perspective of ride efficiency. What do I mean by training ride efficiency? I am primarily focused on L4-L7 efforts (the mix varies, but my primary ride goal always encompasses one or more of these levels), so I measure training ride efficiency as total minutes at L4-L7 divided by total minutes on the bike. I then summarize my ride time by week. Attached is the frequency distribution of my training efficiency index by week for this year. BTW, these are real minutes by level (e.g., L4 is >= 10mins duration and >=91%FTP) and not simply a frequency distribution of the watts data series.
My basic observation is that even though I work very hard at being efficient in the use of my training time, I can make still further progress. My overall ride efficiency index for the year is 44.5%, but some of my weeks were as low as 10-20%. My trainer rides are my most efficient rides, hands down. Even when I go out for a long L4 (I have the perfect route just 5mins from my front door -- 15 miles long, 1.6% avg upgrade, typically upwind), it's only a little more than 50% efficient due to the long descent back home. I think I can push my road rides up to ~50% efficient and my trainer rides are always >75% efficient. So, I think I can milk at least 10% more efficiency out of my rides. 10% is a pretty big deal when applied to >20,000 minutes year-to-date.😄