We all recognize a well-paced rehearsal when we experience one.
What does it mean to create a well-paced rehearsal?
I think one of the most important things is to plan in advance for variety: hard work on some individual pieces or sections of pieces, run-throughs of more familiar music, alternating faster with slower music (or music with a jubilant mood with something that is more restrained), music that is easier vocally with music that's more demanding. Vary how much time you spend on each piece as well: one piece might have a run-through, one might have just a short rehearsal on the chords that are still not secure, while others get extended rehearsal time. You get the idea.
What's best to start the rehearsal? Think about what will get the group focused. Do you open with vocalises? Do you connect vocalises with the first piece you'll rehearse? What music will get the choir involved?
Where should the hardest work of the rehearsal go? I'd suggest somewhere around the "golden mean" (meaning a bit more than half-way through the rehearsal), so the group is well-warmed up, they've already done some work to prepare them, and are ready for a challenge. Too early and they're worn out for the rest of the rehearsal. Too late and their focus or energy might not be as sharp. If you have a longer rehearsal and take a break, I'd suggest before the break--that gives them a chance to recuperate before the last half of the rehearsal (and when I say "half" I don't mean literally--the first "half" should be longer than the second!).
End with something the group will enjoy--it can be a favorite piece or simply the chance to run through some music without stopping. Send them out of the rehearsal feeling good about what they've done and wanting more.
Speaking of stopping, too much stopping is frustrating for the singers. Make sure that the kind of detailed work that requires lots of stops and starts doesn't go on too long and you give the singers the relief of being able to sing through a complete piece or section.
Your literal "pace" (how quick the instructions with little downtime for singers) is also important. Too quick a pace can be as enervating as too slow. Even that can be varied through the rehearsal. One piece can be rapid-fire and another give them time to breathe (your instructions can be at a slower pace). If I've been working them very hard (although ideally I don't want to talk too much--quick instructions, then back to singing), a short mental break for them to slump and listen can be helpful: talk logistics of your upcoming concert, or take the time to explain something about the music or composer (if it doesn't go on too long!) can be helpful--a mental and physical "mini-break" if you will.
Of course, different groups (age groups, experience level) will have different tolerances for hard, detailed work. Given your group (and you know them best), you want to push the boundaries of how long they can focus and concentrate, but not so much that they get frustrated. But improving their capacity for hard work is important.
But even the same group will have days of high energy and days when they can't seem to focus. Sometimes you can push, cajole, or charm them into working at their peak level after a slow start. At other times you may recognize that it's best that day to ease off a little, change your rehearsal plan, and spend more time on music that's easier, less time on detailed rehearsal and more time on run-throughs or favorite music for them to sing.
The goal is to pace so as to get the maximum out of your singers in the time alotted--but to keep them motivated and loving to sing. I still remember a line from Brock McElheran's wonderful short book on conducting where he says (I don't have it in front of me, so this might not be exact), "It does no good to memorize long lists of baroque ornaments if no one wants to play them for you."
I try to remember that.