Howdy sort-of neighbor! I wish I had your power and w/Kg numbers I started a new commute in March, went from 4 miles, mostly W&OD to slight rolling 10 - 12 miles 1-way depending on how much of the traffic on Centreville Rd between Herndon & Chantilly I want to deal with. What I did:
- 2x week, usually Tue & Thur: make at least one of the 2 rides that day an interval or steady-state session; the traffic lights usually force me into a 3-6 min interval with a forced rest at the stop lights. Adjust by a day if weather or work is going to get in the way. If I'm on the commuter bike (no power data), go by feel or try to practice some other aspect of my riding.
- Sat/Sun: Intense indoor session, or since early April, long ride; throw in some hard extended efforts or hill climbs depending on route. If I go Saturday, Sunday will be an easier ride of off day depending on family obligations.
- Other days: take it easier on commute unless I feel particularly firksy that trip.
- Because I am a numbers fiend, I started tracking data on each commute - time, distance, averages for HR, power, and after I load it into Training Peaks, their estimate (power or HR-based depending on the bike) for TSS. I also started plotting the TSS data and calculating training loads (per Coggans & Howard's Training With Power, algorithm is also explained on the Training Peaks site). I believe the pay version of TP calculates this for you, but I didn't want to spend that much at this point. Golden Cheetah is free (as in beer) but only calculates a training load if it has power data. TP will estimate a training load from HR or avg HR data if that's all that you load in.
- It took me about a month to get accustomed to the new commute, there were some days I really felt beat up (and some of the March-April weather didn't help, either). Some weeks I did only one targeted day. I feel I can handle 2 hard days/week plus a big weekend ride OK now.
- The other days I can work on cadence, pedal technique, or just enjoying being on the bike (even if it's in DC area traffic!) and not paying gas, parking, transit fares, car tax, depreciation, ... You could, for example, work on spinning in a lower gear if increasing cadence is a goal. Just decide on a purpose for the day's commute, even if it's saving $$ for more bike swag.
- If more intense exertions are what you want to work on (bridging gaps, attacks, sprints), use traffic lights/stop signs as break points for going all-out on the hard days.

I've noticed a slight weight loss which is nice to get but wasn't an objective, as well as a steady decrease in riding time/speed increase on the commutes. Other than as a way to get the "training" aspect of the ride back towards an hour, I'm not considering lengthening the commute; I'd probably add to the route going home, just because I have more flexibility after work than before. My commute takes me past where one of the local weeknight group rides starts; if I can figure out where to leave my stuff for 2 hrs, I can use that for some extra hard riding.