I took 23hr 48min for my BG. I am a very average fellrunner who really comes from a fellwalking background. My rule was to run the descents where possible. There are relatively few flat bits but generally they need to be run. After about halfway however the definition of "run" changes to something akin to a "shuffle" at best. On the clockwise round (which I would always recommend) there is relatively little running after Stake Pass, apart from after Robinson when fortunately for me I had plenty of time and therefore could walk most of the way back into Keswick (except most of the downhill bits).
One of the keys to a successful BG is to not to go off too fast on the first 2 legs where the ground is generally pretty runable. Certainly you shouldn't be running any of the uphills unless you want to do it in less than say 22hours.