There are a lot of positive things about trails too, it's just human nature to point out the bad while assuming the good is common knowledge. I've ridden rail to trails around the great state of Michigan and have, under the right circumstances, found them to serve a very useful purpose.
For instance, US Hwy. 10 is nothing but a high speed strip of ashphalt through nowhere that has very minimal shoulders and a surface in such poor condition that it will jar the fillings out of your teeth if you're riding on high pressure tires.....but..... there is a rail trail that runs parallel to it for about 80 miles that just may save your life or at least your teeth.
That said, I suggest you follow the RTT conservancy
guidelines to take advantage of a proven success plan for this type of endeavour, it will help you avoid misguided energy being wasted.
2003 Iceman Challenge - 2:34:55 - 897 / 2,000*
2002 Iceman Challenge - 2:39:23 - 1093 / 2,186
2000 Iceman Challenge - 2:49:18 - 1516 / 2,153