- The base Unity measurement unit is a meter.
- A terrain that is 1600 units large measures 1600 by 1600 meters.
- Decimals are centimeters: 2.35 means 2 meters and 35 centimeters.
- Unity is an exclusively metric affair. Always meters, not yards.
- The CF has a tape measure that you can set to meters or yards
- And, of course, 1 meter is 1.09361 yards and one yard is 0.9144 meters (or 91.44 centimeters)
I always start my fantasy course designs (I prefer the word "fictional", sound less like Frodo and Sam stumbling onto my course) with a 2D Photoshop overview. Once I have decided on the dimension of my course there, it's easy enough to determine the distances in yards. I create a Photoshop level with a special 600yd ruler that I use to lay out each of my holes. Sometimes even add a grid to my overviews - in 10 m increments - to later "see" on the terrain what the distances and scales are like.
If you don't want to pre-desgin a course, still the best idea would be to create a grid, be in in yards or meters increments, that gives you a basic notion of space and distances.
If you don't want to prepare your job in Photoshop, you will still nees points of references. There are freeware scripts for distance measurement in the Asset Store for you, but these are a bit cumbersome and do not give you a visual feedback. If you want to go in there without any helpers, perhaps plant some of your own and use what is already there: Create a few "cube" game objects, for instance, and make them 10 or 20yds in size, then line them up in a chain and move them around to get an idea of distances.
You're absolutely right to feel concerned about this issue.