How do I control the duration of my notes?

I have been playing the dulcimer for a couple of months now. Actually, this is a questionable use of the word "playing," but I am having fun. I have recently been working on strumming and have been practicing it with a metronome. I am trying to develop a consistent strum. I am having problems trying to figure out how to control the length of a note. Let's say that I am playing a measure in 4/4 time with one half note and two quarter notes. If I am strumming at a consistent rate, it seems to me that all of my notes will be the same length. How do I vary the length of the notes?

Thanks,

Wright