There are about a million reasons for feeling too pooped to start. Age, hormones, mental fatigue (even though you feel mentally ready, sometimes there are sneaky subconcious things that make you feel physically tired).

Some of the girls here have talked about just getting on the bike for a brief spin around the block to get the endorphins flowing, which paves the way for your body to kick into gear - this worked for me. Don't tell yourself that you have to ride 100 miles, just around the block to see how you feel. This is all assuming you don't have an actual physical reason for fatigue, like anemia or chronic fatigue, etc.

Here's hoping you can enjoy your new bike soon!