Before continuing in this lesson, go back and read the section "Visually intuitive note durations" in the Complete Summary. One of the key points is that note duration is expressed in terms of beats and displayed with the use of tic marks on the staff. The other key point is that the rate of the beat is fixed over at least a short span and for most beginning tunes, over the entire score.
Sometimes a score will precisely specify the rate in beats per minute. Other times, it is less precisely specified with descriptive words such as "slow", "fast", or even "blues tempo". There are a zillion of 'em and you just have to learn by experience or intuition what they mean. There's definitely room for interpretation. And often there is no specification of the beat at all and you just choose whatever you like (of course you can always do this anyway, it's your piano).
You can keep the beat by counting to yourself, tapping your foot, or using a metronome. Of course the latter is the most accurate. You may have access to the old mechanical type but if not you can easily find an app for your device. Whatever technique you use, it's probably best to start out really slow. Gradually speed up as you gain proficiency.
Let's now revisit Ode to Joy. Hopefully, you downloaded it in the previous lesson but if not go get it from the Downloads page. In that lesson, the focus was on pressing the right keys. Here, the focus is on how to press them at a constant rate. You can watch this video on YouTube to see how it sounds and is played. It starts off with a warm up exercise of playing the 5 notes in the range of the C position with the right hand. In Ode to Joy, all but the last note in measures 4 and 8 are 1 beat long and are of equal speed and duration. To play the last notes in measures 4 and 8, hold down the key for the length of two beats before releasing it.
Notice that this tune is shown on the treble staff which is to be played with the right hand. So play this now while trying to keep a constant beat. Start with a slower beat rate, AKA tempo. Then play it again with a faster tempo as you are comfortably able to do so. As in the previous lesson, remember to visualize your hands on the staff.
Next, try another tune named Aura Lee (or, Aura Lea). This was written to be played with the left hand on the Bass staff; notice the octave numbers and the left hand fingerings are different from the treble staff. While still mostly one beat notes, it also has two and four beat notes. So same drill, different hand. And here's a video of it. Again the intro is practice playing the 5 notes in the range of the C position with the left hand. You can get MusicXML file from the Downloads page.
If you liked the movie Trading Places (1983 with Dan Akroyd and Eddie Murphy), you might get a laugh in seeing a parody of this tune. If nothing else, it'll help establish this tune in your head which is always key to getting to playing it so it sounds right.