Long Distance Competition/Ultracycling, Randonneuring and Endurance Cycling - Cycling: training and tapering

I have been training for an IM triathlon where most of my time is spent on the saddle. I have build up and my weekly long rides have been over 50 miles for the past three months. Some weeks vary but I get anywhere from 130-200 miles in a week. More on the shorter side. Currently I have done a century a week for the past 5 weeks, with some 12 x 1 min(4 min recovery) sprints, hill work, and TT's. The speed work, hills, and TT's however do not put on the miles, maybe only 15 per workout, usually putting my weekly mileage at around 145.

My race is on September 7th and I do enjoy getting out and doing 100 miles every week. My economy seems to be improving, but my speed has not picked up at all(solo around 18.4mph, saving energy to run after). I know it is too late to add speed to a distance that long but what will be some things in the off season that I can do to improve it?

Also when should or how should I begin tapering and reducing the miles? Keep putting in long days on the bike until the last week? 2 weeks? Reduce to some 50 or 75 milers? Drop the intense speed and hill workouts?

Thanks!

If your goal is tri's, you actually might be better off in the tri forum or with a pro trainer. After all, you need to balance out your endurance resources across all 3 sports, not just cycling. You may also be able to modify your position, setup, and gear to get more aero. (I assume you already have aero bars...).

That said, in general you need to do interval training. Since you clearly have your base, something like 1 hard day a week, a bunch of days at pace, and some rest days will improve your performance far more than repeating the same level of effort day after day. A trainer will give you a much more precise and personalized training regime, of course.

Also, in general you want to taper (and carbo load) a few days before the event.