The most common thing we hear from adults is a version of: 'I'd like to, but I'm out of shape.' That's a reason to start, not a reason to wait.
Month one: mechanics
You'll learn stances, basic strikes and how to move without losing balance. Your conditioning will lag behind your interest, and that's normal. Scale drills, take the breaks you need, and keep coming.
Month two: rhythm
Techniques begin to link together. Class stops feeling like a series of unfamiliar movements and starts feeling like a workout you can follow. Most people notice mobility improving before they notice strength.
Month three: progress you can point to
You'll have a first belt in sight and a clear list of what it takes. This is the point at which martial arts tends to stick where gym memberships don't — there's a defined next step and people who notice when you're absent.