I used to always head for hills, and in fact, for anything longer than 60 mins, I always head for the hills (we've got great mountain terrain in Norcal) but I'm finding that for under 60 mins, I get a better overall workout with the flats.

My typical hillclimb workout on a local climb that's about 6 miles nearly up the whole way: 40 mins up, 20 mins down a curvy descent.

My typical flat route: 55 mins of fast solo riding, with 5 mins of lights and stoppage.

My HRmax is higher on the hills, but I get a more consistent and longer Zone3 (have been using HRM) hard effort on the flat route, which makes it feel higher yield for me in terms of a workout. On the climb route, the downhill, while fun and good for technique, yields me zero in terms of fitness, and comprises 1/3rd of the entire workout.

Just surprised me, as I've heard from almost everyone that given hills vs flats, the hills > flats for workouts, but I'm definitely finding that's not the case for shorter workouts.