I try to leave my new strings strung for at least overnight, and preferably 24 hrs before I serve them. If I can't wait as long as I'd like, I serve them so the twist of the serving will be tightened as I twist the string to make it shorter. The string will always stretch a little (or lot) more, and I've never had to lengthen a string once it was served-only made them shorter.
Your fingers will get stronger as you make more strings, and your technique will get better and it won't get as tiring.
I trim the tag ends off with a pair of fingernail clippers I keep in the string box for that reason. It seems I always have a few, some strings more than others. Then I wax the string well and rub it with some old canvas pieces I have or some leather. That will form the last of the tag ends into the string, improve the appearance and lengthen the life of the string.
It can become addictive, and I've found myself making strings for friends and extras for the bows I own.
RonP