Help out a newbie

Use higher gear ratios, which means using smaller cogs in back and/or the bigger chainring in front. Also, another great way to get a tougher workout is to ride harder.

I use Strava app, and set goals against segments on the map. Makes you push for something, sounds like you have the conditioning, just push harder. Also, did you get fitted properly?

Yeah I believe I was fitted properly. I tried out a whole bunch of bikes and this one (Trek FX 7.5) just fit me like a glove.

The only problem seems to be that because it has such little resistance (weight, rolling), I'm breezing through hills and going fast too easily to get a decent workout.

I remember when I first took up riding on my old bike again, the tyre pressures were way below ideal and I'd come home and literally fall into the pool just to cool off. Now with fully pumped tyres on my road bike, I come home with a mild sweat lol.

What I'm basically asking is if mountain bikes are better for exercising in the suburbs than road bikes because of their inefficiency (because obviously I can't be doing 60km/hr on shared cycle paths).

Gotcha. I used to ride my mountain bike on the road, it's a much harder workout obviously - but the idea of the road bike, and you don't need me to tell you this, is distance. I could never go more then 10 miles on my mountain bike without shoulders/back hurting. 10 miles on a road bike is a warm up. Go further, and faster!.

Same here. I put slicks on the mountain bike to ride the road before getting my first road bike. My first ride on the road bike felt like gliding using a fraction of the energy.

"How do I get a good workout on a light and efficient road bike?"

Strap an old car battery under the seat?

"What I'm basically asking is if mountain bikes are better for exercising in the suburbs than road bikes because of their inefficiency (because obviously I can't be doing 60km/hr on shared cycle paths)."

Every now and again I run into a local farmer that rides a mountain bike. I can't believe how hard he has to work just to hold what I consider to be a 'talking pace' tempo. Mountain bikes, especially the cheap, dual suspension department store models, do offer loads of Watts into every ride!

Yes, 35 MPH on a multi-use cycle path would be suicide in my area. You might have to stretch your legs and find yourself some secondary/tertiary roads that will give you more miles, more challenging terrain and that tougher workout (as opposed to trying to do 20-25 MPH through your suburban neighborhoods).