33 Mile Road Race - Training Tips?

You have a month to go to that race so use that time to improve your performance.

If you can easily cover 33 miles, at 20-25mph and you're not feeling tired then you're in good shape as it is.
(this is what your posts says) for this specific race.

20-25mph is a good racing speed. If I can offer advice it might be beneficial to work on doing intervals to train your system to work harder for short/intense bursts.

Breathing? Breathing though your mouth as opposed to your nose is much more efficient. Of course, if you're seen gulping air down this might make your opponents think that you're struggling.

If you have been doing 20-40 miles a day you are probably in decent aerobic shape for the 33 mile event. I would continue with at least one weekly ride at the top of your mileage range. With so short a time out, Limericks advice on the intervals is where I would go. Longer intervals of 3-4 minutes with twice the length of rest between will not only help raise your speed ceiling, they'll also help with that climb at the end. With a month to go I'd do these workouts twice a week to the final week. And then shorter intervals of 30 seconds and not much rest between sets (30 secs) to help deal with processing lactate in the final 2 weeks. I'd do 6 reps of these (30 secs on, 30 off) in the 2 or 3 weeks before, replacing one of those other interval sessions. Dunno what your age or general health is but I do best limiting the pain cave sessions to twice a week, assuming one of the weekend days has some spirited riding.

That climb sounds like it's not only going to determine the podium, but also provide 3 rider selections along the route. Try and duplicate the effort in training. So on your 30-40 mile rides, make sure to include 3 hard efforts that will last roughly about as long as the climb, and then practice dealing with still being able to keep your speed up after the effort.

With limited info to go on, lower back pain can allude to a bike fit issue. However, a month out from the event (where training intensities are usually highest) is not the ideal time to be fiddling with position. Maybe some stretching after rides? An ice pack on the culprit area for 10-15 minutes after the ride can reduce inflammation and possibly help (can certainly never hurt). Heat on the other hand can make things worse.

Re: your breathing - Lady Gaga says it best... Pa-Pa-Poker Face. My favorite time to hit the gas is when I hear the tell tale labored breathing of others. I did a race two years ago wher some poor sod forgot to turn off his HR monitors ceiling alarm. It went off midway through the climb. A second later the pace picked up dramatically. Whether the almost simoultaneous attack was coincidence or not, you are doing yourself no favors by advertising your oxygen debt