Welcome to the forum.
The only experience of boating I had when I started was childhood holidays and it was also at the start of lockdown, so no courses were available even if you wanted one. As has been mentioned handling a boat is the easy bit but if you lack confidence your best bet is just to set yourself a cruising goal and get out there... for, reasons, I had to move my boat half way across the country with zero experience... no-one died, only had one significant bump and it wasn't into another boat (might have been less damaging if it had been). Solo boating is easy... 9 times out of 10 other boaters will help you when needed but it's always polite not to assume this will be the case. That said, if you have the time and money to do courses there is certainly no harm in it. I may well have bad habits that have never been questioned.
57' is probably the most typical narrowboat length. If you don't need the space a shorter boat can be handy for turning where other boats cannot. Mooring and your licence are also charged by the foot, of course.
Maybe I'm weird but winter is actually the best bit as far as I'm concerned.