Tardé una hora en escribirlo
So tardar is, according to our dictionary,
- to take,
- to be late.
But the take really means in the sense of time only, isn't that right? So it really means "to take time" and not just "to take" because it isn't used without a time measure following it unless it's in the to be late sense. Isn't that right?
It took (the time of) one hour to write it.
Also, the show said "Tardé a una hora en escribirlo" but I think the "a" is wrong there, ¿Que dicen?
You are right on all counts. Really nothing more to add! Perfect!