One of my favorite workouts to prep for a race is a tempo block followed by surges. This is a great way to simulate the efforts required in a road race, when you are already dealing with fatigue when the racing starts to get serious. I started working with Nate Wilson at Catalyst Coaching in the fall of 2015, and he had me doing these intervals pretty quickly once I picked up training after the offseason. Here's an example of some of the numbers I aim for when I do this workout:
15 minutes at 330 watts
5 x 30 seconds at 500 watts, 30 seconds easy (5 minutes total)
The key here is to make these accelerations, not full sprints. This is a mistake I made when I first started doing these. This is a workout to prepare you to make it to the finish line of your goal races. You want enough training stress to stimulate improvement, but not to smash yourself. Save that for the race! I might do a couple sets of these during a long ride, or sometimes that is just one of a several different sets of intervals. A pro tip for these kinds of very structured workouts: you can write yourself a "cheat sheet" on a small piece of paper and use clear packing tape to attach it to the stem where it's easily visible. Or, I usually try to save a screenshot of the interval instructions on my phone, in case I need to refer to it quickly.
I sometimes call these kinds of workouts "leg breakers", if you do them right you might not feel too bad during the efforts, but you will definitely feel them walking around afterwards!