Unless you are already in crazy good shape, I have to say I would agree with cpp214 and Gabkicks, though Sick Cylinder has a good point about taking a decent length brisk walk.
I think the biggest factors for how much of a workout sim-racing can be is your equipment set up, and the length of your session. Potentially the amount of aids you use (abs, traction control, automatic gears,etc.) could add/detract.
As stated previously, a Direct Drive wheel (with moderate to high ffb) and stiff pedals will give more of a workout than, say, a logitech DFGT set-up (the first wheel and pedals I ever had.)
As Gabkicks stated, doing longer races can be quite tiring, even in single player. Do a full distance f1 race at Monaco with competitive AI and damage on. I can pretty much guarantee you will be sweating after a couple laps, and still have 75ish laps to go.
Adding into it, the mental side of sim-racing beyond just driving around the track (and depending on the car/sim used) can include adjusting brake-bias, differential, ERS, fuel consumption, tire selection, tire wear management, abs settings, traction control settings, using DRS, wing adjustments, changing pit strategy.
As with anything you do, the more you do it, the more used to it you become, the less physically and mentally tiring it will be. If you're new to sim racing, there is potentially a lot to learn depending on how far into the simulation side you want to go. There is also a lot of physical adjustment/learning to do.
