If you are used to running, then extra training on the bike will probably help overall. I was a club runner for many years before I tried triathlon. I did a whole load of running PBs the season after starting triathlon training. The extra cardio-vascular fitness more than made up for running fewer miles in training.
I do think it's much harder for someone coming from a cycling background to build their running up to a good level than it is the other way round.
I do think it's much harder for someone coming from a cycling background to build their running up to a good level than it is the other way round.