Question on warm up

[Hound92]

First of all I am a late in life swimmer. When I start out during a 100m warm up everything seems off and I can't get into a good 3 stroke rhythm (breathing every 3 strokes) without struggling and running out of air.

Then I switch to drills. After 20 min or so of doing drills I'll do a longer swim and can go 200, 400, 800, etc. with complete ease. As a matter of fact if feel like I can keep going indefinitely. Is this common?

Appreciate any responses from you experienced swimmers.