I'm NEVER one to knock a flat ride but bear in mind that on a truly flat ride, one tends to not shift one's weight around as much, get out of the saddle, etc., as they might on a hillier ride. So a super flat double can be hard on the lower regions than a hillier one. A moderate climbing route may be a better 1st double choice....

Just a thought.

- MP, who loooves a flat, fast ride in spite of her moniker