Hi, I did my 1st sprint duathlon this year.
I was in a similar position of running half marathons & being able to cycle 30-40 miles.
I didn't have the most specific training plan. I just continued my general cycling/running training but just added a few brick sessions to get used to the feeling of running straight from the bike.
I just used google and picked a distance. Think it was 1m run/3m bike/1/3/1
Then about 3-4 weeks before I had a go at the race distance 5k/22k/5k