I am looking for some methods of cross training. Can't access a pool nearby so I'm trying to incorporate biking into my training regimen. (If not, I can still run, I just like to change it up a bit, otherwise running becomes dull) If I'm biking around my neighborhood in a mountain bike for 20-30 minutes, will there be any benefit? Or do I have to bike for 45 minutes to 1 hour or longer to reap benefits from biking?