I will speak from my own experience:

When I was a kid (9 years ago) my mom bought me a walmart mountain bike (Mongoose I believe)...I thought it was the coolest thing ever, I loved it, and for the 2-3 mile rides I did it was perfect.

Then through Boy Scouts I started preparing for a 6 day 180 mile bike tour/camping trip. During the longer practice rides, my chain would frequently come off, gears wouldn't shift correctly, etc.

Then my parents invested in a baseline Gary Fisher Tiburon for $300. Looking back it seemed like a lot of money at the time, because my wal-mart bike was about $80-100. There was a world of difference between the Gary Fisher and the Mongoose. The GF was easier to ride, shifted smoothly, and literally never had a single issue with the chain/shifter/derailleur. I took the bike on two 180 mile trips, and then used it to commute while in college, never had a single issue. I was heartbroken when the bike was stolen during my Junior year of college.

Then I started using my brother's baseline Fuji Finest-AL Road bike...he got it used for $100. This last summer I rode on it 30-40 miles a day...the derailleur broke during one of the rides, and the chain has some issues on the lowest gear, but overall smooth and reliable.

Shortly before that I purchased a Brompton Folding bicycle...and that is my most expensive bike. I commute 20 miles/day with it...never had a single issue, it is solid as a rock, fast, and smooth.

So, a Wal-mart bike will be more difficult to ride, and may have more minor mechanical issues (chain coming off, improper shifting, etc), than even a slightly more expensive bike...but for the amount you are riding it should be perfectly fine.