They’re usually different sizes. But you can buy Mt Bike slicks that can inflate to 90 PSI. (Mt Bike tires are usually in the 50 PSI +/- range.)
I got the Craigslist Trek. It fits me nicely, and is in fantastic condition. I did buy a new seat, because that hard thing that came with it is crap, but otherwise, perfect for a starter bike, I think. I paid $180.
I rode it up and down my driveway a few times (driveway is about 1/4 mile, gravel) and I’m already sore. So much for being in pretty good shape! Seriously great workout, I can see why all those biker types are always ripped.
I think I’m going to take it to the trail for baby steps tomorrow, so I’ll let you all know how it goes. 
Thanks again, everyone!
The Rock Island Trail is the part that runs behind my house. They’ve got it done through Chilhowee, which is a few miles from me.
I think you made an excellent choice; the bike you bought looks perfect for what you want to do, and the price is really good.
I’ve found that if you’re rear gets pretty sore after the first ride or two if you have not ridden in a long time, but the you’re fine.
That looks like the right bike for your purpose. I think you can lock the front suspension by twisting those knobs on the top of the forks.
Looks in decent enough shape as far as I can tell - might want to have the LBS take a look at it just to be sure.
Brian
It actually doesn’t have front shocks, although it looks like it does. It’s just adjustable forks.
Those are shocks. They are adjustable, I cannot tell if you have lockout ability though. Based on your answer, I’m wondering if they may have something wrong with them - perhaps you should have a mechanic look the bike over.
Fortunately you have an RST fork and not SR Suntour (Trek had to make a massive recall a couple years ago on those).
Ha! You are correct! They are shocks, they were just turned off. Neat.
You should be able to adjust the suspension in your seat post also if you feel like getting a little adventurous:)
Well, would you look at that! So, both the seat and front shocks were locked down. Why? Why would you not want shocks? The girl I bought it from used it as a street bike, if that’s relevant.
In general, suspension slows you down on flat surfaces - instead of all the energy going forward, some of it goes up and down. If you are mostly riding on the street, you will go faster if the suspension is locked down. Probably for trail riding you would want somw suspension for comfort.