First time on a bike in 15 years so forgive my ignorance here. I got a Trek 820 to see if I stick with it and try to get back in shape. Figure I'll upgrade if I become addicted. Question is, since I'm a ways off from trail use I'd like to throw some road friendly tires on it. What size/type tire should I look at? How skinny can I go on the stock wheels? Recommendations?