I have an Ariens 42 in ride mower with 17.5 BS engine.  It has been running fine that is until yesterday when I didn't see the remains of an old low cut stump while mowing some tall grass.
When I hit the stump I immediately shut off the engine (or maybe it shut off itself) pulled the tractor away and inspected the underbelly to find no hungup wood pieces and the blades looked intact.
The mower started up easily but the engine idled so high that it scared me so I immediately shut it off.  After looking over again, I restarted it and again same high rev-ing engine at idle that I shut it off again. I then rolled it back onto my trailer where it rests.
Can anyone suggest what the problem might be/ how I might approach fixing it. 

Governor may have broken with the impact.

Is this something a novice could repair easily enough or does it require someone who knows their way around mower engines?
My Ariens has Briggs&Straton 17.5 hp motor model 31C707
 
Thanks for the help you are providing

You could watch some YouTube videos on governor gear replacement and see if it is something you want to tackle. You could call a small engine shop if your not comfortable doing it.

The engine need to be pulled and the lower sump cover removed.  
You can see the yellow/goldish gear in the pic's. That likely broke.
 

 
 

I watched part of a utube video on the internal govenor on the engine.  
Removing the engine seems easy enough 4 bolts, couple wires unplugged and drop 2 pulleys.+
I will have to watch more to see if getting to the govenor is difficult???

Once the engines off, clean the crankshaft of any rust where the seal will slide over.
 
Once clean simply remove the side cover bolts and the sump is off.
 
Re-installing is a little more involved as you have to align the oil pump rod up.
More on that later should you proceed..
 
 

question: how many hours would it take to do this repair?

Engine out, couple hours, taking your time..
 
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Looking at parts for this project; it is hard to find OEM Briggs & Stratton.
Anyone use 'Chinese'-manufactured parts which is what seems most available on-line?
Are they dependable??