I'm about to upgrade out of Cat 5...question...
 

my local club races are usually 30 miles for the Cat 5 group. Cat 4 jumps up to 36 or 42 miles.

So far, 8 races in, i find that i'm just not as fresh for the final sprint as i want to be. I'm finishing 4th and 5th.

my usual morning training rides during the week are 18-24 miles, depending on how much time i have.
Should i focus on going for longer rides when training (at the same intensity) to build endurance, or is there a better way?

You need to do at least one long 3-4 hour ride a week. You don't have to go hard on the long ride if you are doing high intensity on your shorter rides but you do need the hours in the saddle to build your endurance.

It's normal to be "not as fresh" for the finish in races. 30 miles is about as long a crit as you'll see if you upgrade to Cat 3. I understand you're doing the Park races but they're still hard, lots of longer efforts, not much resting.

A lot of people will say that you can ride your shorter rides more intensely. As it is you're going 1/2 to 2/3 of race distance in training. Throw in a couple longer rides each week, like 30-36 or even 42 miles, maybe twice a week. Don't forget to take a rest day, preferably at least two days before a race or hard ride.

G - instead of training in CP, you should roll out to PP for some training. I live in Brooklyn and when I do training in CP, I get in about 40 miles before work. 10miles to CP from my house, 20 miles of riding in CP, 10 miles back to my house = ~40 miles. I use the ride over for warm up and just spin high rpm. On the way back, I do intervals part of the way and then cool down... And I'm at work in Manhattan by 9am. So, if you were to reverse what I do and train in PP for a day or two, you'd be able to get in ~40miles before work. It's an early morning, but a great way to get in miles. I'll see you on Wednesday in CP and if you are interested we can coordinate some PP training.