The most important things you get from your run up are rhythm and speed. In order to do this you will need practice.
In the outdoor nets, start at the popping crease and run away f from the bowler, taking your delivery stride.
mark this point.
Practice running in off this length with someone watching for front foot no ball. Adjust your start point to take this into account.
Now measure this point in walking paces from the popping crease and remember it. It is your run up and is the distance you will always use.
Now you know the distance, practice, practice, practice to get rhythm.
You need to spend a long time getting comfortable with it.
The practice leads to you knowing that your feet will be okay so you can concentrate on the delivery.
As a formerly quick bowler, this us what I was taught and it works.
Later you can learn about shortening your run up, if necessary.