A book I bought years ago (still available) uses a point system to rate various aerobic exercises. The points were assigned based on objective lab measurements. The book is probably in many public libraries: Amazon.com: Aerobics Program For Total Well-Being: Exercise, Diet , And Emotional Balance (9780553346770): Kenneth H. Cooper: Books
Example from tables in back:
Walking/running for 2 miles in 30 to 40 minutes: 3 points
Cycling 6 miles for 24 to 36 minutes: 4.5 points
Cycling 6 miles in over 36 minutes: 2.7 points
The book has recommendations on how many points you would want to get to be in good shape.
Might be a good book to buy used, it's cheap.