1. Reduce the variance! 40w on such a smooth flat course is too much! it gets even worse after the turn. on a course like that just dial into a natural cadence ( whatever that is for you.) and pick a gear which gives you the right output, and focus on holding it!
2. Start slow finish-fast you can't hold 320w for 20min so start just below 20min threshold more likely to be 290-295 and then nail the last two miles!
3. You are lucky to have such flat courses! the "flat/fast" ones near me in the UK have at least 100m(330ft) of elevation change in them.
The issue with keeping a tighter watts is when I'm on the areobars I cannot see my Garmin since its on the stem. What I do is set high and low alarms and just listen and adjust when I hear the alarm go off. I either need to figure out how to tighten the range or figure out were I can possibly relocate the Garmin and see it when in the aero position.
I would love to see links to others members races. Please post up if you have them. Or even power workouts.
Being able to see the numbers is a big issue! TTing is my forte, but for less than an hour I work on feel ( mainly cos is prefer to use a disc and can't afford to upgrade from a wired PT..) I've created many mounts out of spare bits and bobs +cable ties between the aero bars for testing training and 50mile TTs. Anything to get the garmin in eyeline will work.
Will be doing quite a few 10mile TTs in the next few weeks for testing new positions so will try to post them. I'm sure they will break my rules, but at 10mile level you can bend the rules a little over little rises and head windy sections once you know what you are capable of with very smooth power delivery. I will try and post them anyway!
Oh and finally being 55 is no excuse, my coach is about that age and can still put out 340-350w FTP.
He finds it hilarious to kick 20 something year olds backsides on the local chaingangs!