Thanks for that, a lot of people I cycle with have moved to some extent from running to cycling due to knees etc., but quite a few still do some running.
At one point a few years ago I was doing both quite regularly and although only running twice a week was building nicely to a 1h45 half as my endurance levels from cycling were high. Annoyingly injured my knee (unrelated to either) two weeks before the event and that was that!
My trouble is I'm a bit one paced so struggle to go slow or fast...my running intervals at the moment are around 5:15-5:30/km which is what feels comfortable...full gas I'm probably 5:00/km plus or minus a bit!