For sure go running. It’s good for you in every possible way.
I mix up running and cycling and it’s a good mix. I’ll generally do about 6 hours a week on the bike and run about 30 miles, mostly on trails. People say that to become a faster cyclist you must ride more to the exclusion of other sports, which i guess is true if they cannibalise the time and you are aiming for the top of your sport, but as a M49 who rides and runs for fun I love being able to oscillate between the 2. They complement each other as mutual cross training, they toughen you up in different ways and they both provide an outlet depending on your mood. Sometimes you really don’t want to go sit on the trainer, so a 5 mile run in the forest is perfect. Running is very time efficient as well - it’s easy to nip out for a quick 5k in a spare half hour.
Take note what others have said if you are totally new to running: Your connecting tissues take a long time to fully adapt to the new strains. So although you might easily be able to go and run 5 miles several times a week, using all your cycling aerobic fitness, your legs will soon start to really hurt. So start on low volumes: 10miles a week, 12 miles a week… My first year’s running (I started aged 44) was littered with injuries, now i can run 40 miles a week with no problems…