Bicycles are much different than motorcycles. I rode thousands of miles on road bikes as my main method of transportation over the years. Not many skills transfer, unfortunately. 400-600 cc, singles and twins are gonna be the sweet spot for transitioning from beginner to intermediate rider.

I'm also not sure the course will allow you to bring your own, but worth checking to make sure. They usually have sport, cruiser, and standard bikes to choose from, and they know they're in good condition and appropriately sized.

I rode a 50cc scooter for 5k miles before I stepped up to a proper motorcycle, no course, and was way over my depth when I did. I failed the license test twice before I passed on the third try after many hours of parking lot practice. I had a coworker who had taken the course before purchasing his bike, and when I rode with him afterwards, he could barely get the bike in gear.

My advice: I would strongly suggest dialing back your expectations, it can be a very dangerous activity to engage in naively. Take and pass the beginner course. Read books on motorcycling: Total Control by Lee Parks, Proficient Motorcycling by David Hough are great places to start. When you get a bike, something modestly sized, gear tf up and practice practice practice, parking lots and low stakes streets exclusively for the first little bit. Slowly build up your experience and confidence. Beware Mt. Stupid.

Ride safe.

I was in the same boat as you. Never ridden a motorcycle, but very comfortable on my mountain bike.

Turns out they are not even in the same ballpark.

Have you ever driven a manual car? That would help far more than riding a mountain bike.

My recommendation is to do the course with their bikes. It'll likely be a small 125cc, but it's perfect for learning on. The difference between what you'll learn on, and what you buy really isn't much. You'll come away with either wanting to continue on a bike, or not. Do this before spending thousands on a motorcycle. It's a no-brainer.

If you like it, go ahead and buy yourself a bike. Like I said, it's gonna feel very similar to the bike you learned on unless you buy a very heavy bike (don't do this to start).

The smart decision is to buy secondhand for your first bike, so if (when) you drop it, it's not a big deal. You also don't want anything too powerful. I got a Yamaha R3, and it's fast enough to get started on. Certainly faster then most vehicles on the road.

In summary, do the course first. A 400 is more then enough for your first bike, and your mountain bike experience doesn't really count for anything unfortunately.

On a highway? My 400 was a thumper, and the hills in my area really made it lag. Not necessarily the same for all displacements, however. I borrowed a friends XT250 dual sport recently and it can keep up with my current NX500. A twisty back road on the 400? I had no problems keeping up with my buddies with larger bikes and would often out pace them because I could really whip around corners and it was less crucial to consider the weight of the machine and its inertia.

My SR400 was quite similar to the bikes at the MSF, but my NX500 is very different. I feel like the skills I built on those bikes were fundamental to graduating to different kinds of bikes, and it is an easier transition than you may expect.