I put together a short wheel base, under-seat steering bike in 2012, and rode it over 1,000 miles.
I put together a home-made conversion the summer of 2013, immediately ordered a genuine Cruzbike conversion kit, and rebuilt the bike.
I found I could ride it right away across a soccer field (a bouncy, slow ride). For your first time, it's nice to have a big open space, then you don't worry about direction, you just stay on the bike. I never fell off once while learning to ride the moving bottom bracket bike.
My commute is about nine miles per day, and that was a nice distance for sorting out riding on trails and quiet city streets. Within four weeks I rode rides of 20 and 35 miles, safely and securely, albeit without letting my concentration waver too much.
I found that it took about 500 miles and 120 days before I realized that I wasn't thinking about riding anymore--I was just going on the bike.
Good luck with your future riding!