I got my "Phleb" certification in 2000 (also 49 yrs old) - before that I worked in vet medicine for 25 years so I had lots of "poking" practice prior to switching over to people. I worked in a our local hospital for 9 years. It's NEVER BORING.. but one profession where you can look like an ACE one night...and a FOOL the next. Learn to forgive yourself if you have an off night. Even the best used to struggle occasionally.
Job shadow someone who's SEASONED at drawing OB/PEDS. Those will be your toughest challenge once you're in the trenches, but once you master drawing newborns.. you can draw anyone. AND.. MOST KIDS have a decent antecubital vein. So.. train yourself to look for it.
Congrats on your new profession! I hated desk jobs too...