Only ever riding can give some muscle imbalances that may ultimately cause downtime off the bike. I run to try to keep myself generically "fit". I'm not a pro cyclist. Not even elite amatuer. I have other chores to do so need to stay healthy. I toss in some light weights/yoga to mix it up on off weeks or a run.
I may do a duathlon once a year or every other year.
The issue for a "cyclist" runner is that if you don't have a run background, it will take a long while to build up your run time for workouts so you don't injure yourself or hurt your bike power.
Like, start at 20min super easy runs and add maybe 5min per week. With that being your long run.
It can help you in a pinch on a vacation or work trip not lose too much fitness. Run on those trips and then hop on a gym recumbent and just crack out some 40/20's to hold on to the power.
I'd just build up to maybe a 10k as your longest run so you can reap the aerobic benefits of running long enough to do an aerobic hour (minimum lower intensity adaption time thereabouts). Then do an easy 5k maintenance run weekly.
If that's your thing.