Running to biking is a much easier transition on the body than vice-versa. If you can keep a couple short runs/week, you can prevent losing all the body’s run adaptations. Otherwise, if/when you go back to running it could be a bit more challenging than you expect/remember.
Your legs will adjust to the cycling. Right now your legs don’t have the strength to keep up w/your engine. Give it time. Pick a plan. Be consistent. The initial gains will come fast since you’re new to cycling.