With a draft, 23/24 would be easier than 21.7 solo. It could even be quite a lot easier.
20 miles solo averaging 21.7 is pretty fast. You’ll be just fine in a race.
But, looking ahead don’t get too focused on average race speed. That crit that averages 23/24 can have wild variations in speed and will often be a be made up of 20 mph stretches (or even less) and 27-28+ mph stretches to that average. Its possible there might not be much if any 23/24mph steady cruising.
The main trick to not getting dropped is have the ability to match the acceleration of the pack when they jump from 20 to 28. Miss one of those even by a second or two and you’re chasing to get back on and eventually if that happens too many times you’ll break contact. People often think of getting dropped as being completely exhausted and flying out the back super slow but in races, often a dropped rider can still nearly match the pace of the pack once the attack is over but there’s a gap and the rider can’t close it.
So, a good solo test is how fast can you go for 1 minute in the middle of that 21.7mph solo ride and how often can you repeat that hard 1 minute effort.