If I were out for a 20 mile day hike I'd probably be carrying 5-10 lbs anyway (a journal/drawing materials, camera, maybe some fishing gear, etc.)
Turn it into a minimal overnighter and I only have to add a 2-3 more pounds.
I often put a 25 lb. block of clay in my pack on day hikes for training anyway. I'm looking at doing Rainier and some other big mountains in the near future- given the amount/weighgt of gear needed, training and backpacking exclusively with an UL/SUL pack doesn't serve this end very well.
A few pounds won't change what I can hike in a day, it's all about the circumstances.
I can hike 30+ miles with a 25 pound pack. Safe to say I can also do it with nothing...Unless I go out to do the maximum possible without weight I really have no idea what my weight/distance threshold is.