For me, I am mostly interested in improving my average speed over any given ride, regardless of distance. I started running a couple of years ago thinking the cross training may help (and it's also a lot easier when traveling). I mostly just jog at 8 min-mile pace for 3-5 miles but can certainly go further and run harder. So far, I haven't seen any improvement in my cycling performance, and in fact, the day after I run, some of my leg muscles are so tired that I feel like they are impeding my riding.