Any advice for how to get used to bike riding?

I am in my 30s and just got my first bike (a used, but practically new, Schwinn cruiser). I am getting so out of breath and exhausted after very short amounts of time on the bike. I am active in that I walk a lot—I don’t have a car—and I go to the gym once or twice a week. Any ideas for how to ease into the bike riding thing? How did you do it? Thanks!