exactly OTG

I get on my Worksman bike and I can't go 15 mph without a most extraordinary effort. If I ride at about 9mph I am getting a darn good workout.
If i had to power that thing up a hill, it would be really really tough.
How is that NOT a workout? how is that NOT getting me stronger?
Let's face it, it's more fun to get on my Bianchi and zip around at 14mph without much effort, but I'd be a fool to suggest that a heavy one speed bike is not a valuable tool for getting fit.