i think you could reap some benefits on a stationary bike if you train right. perhaps keep a certain speed for as long as you can without your HR going too high, then the next time see if you can go longer, and just keep doing that, if it's one of those cool ones with the hill programs that can help too, if not you can just adjust the tension settings at random intervals. and the 5 mile thing on knobbies could be to keep the knobbies from wearing down, knobbies wear out extra fast on pavement, but on new knobbies i'd guess you could still get at least 1000 road miles on them. there's a used bike store in my area that sells used knobby and road-for-mtb tires for $10 a pair, and they are like new.