30-40 mins warm up is about right for a race snake, although a racer would actually be pretty good within a few mins if necessary.
As for stopping and seizing up, sounds like muscles unused to high intensity exercise.
Thing is, to enjoy our sport and let’s face it going as fast as possible is how we enjoy it, then you’ve got to get fit, and that can be as simple as riding as often as possible.
Over time you will remember these current problems with a wry grin and chuckle about how unfit you were. Or you’ll remember them as the heyday of your youth (I hope not!).
Ride fast, never impose self limitations, ride when your legs scream and your breathing is ragged. That way the easy days will feel soooooo good. 🙂