Other advice on here great. My response would be to do an out and back run of 20 minutes from home, at which point turn around and head back. Try and noticeably slow down from your regular 5km pace (for the first few times), and ignore (for now) how far you’ve run. One you’re comfortable with that (no rush) do 25 mins out and back.
Once you’re up to 30 mins out and back, you’ll have broken the back of it and be fine.
Most of us can run 10km at our 5km pace; there’s no physiological reason we can’t, it’s just in the mind.
Remember before I did my first half marathon I went out and ran 7 miles from home, turned round and came home. But I did it an unbelievably slow pace. I just wanted to prove to myself I could cover the distance. After that, never did more than 10 mile training runs.
Good luck anyway!