DH and I did a very hilly 27 miler from home today. Thru lots of canyons and along new bike paths that have just recently opened up. Unfortunately quite a few of them are not fully complete and just dead-end so we did a bit of turning around and backtracking. On many of the flatter sections he got me to practice drafting and riding his wheel. WOW - I'd heard you can use 30% less energy that way and now I understand the concept