Hi Barb - I would use your HR monitor and stay within zone 3 during the first few miles and then settle into zone 4 until you get to mile 20-22. At this point, if you can pick it up, do so and run by feel to the end.
If you have a very specific time goal and you have been monitoring pace with your HR training runs, you could certainly try to run by pace. But trust your HR and what you have learned during the training.
Great success to you and let us know how it goes.