building fast twitch muscles in your 40s

How long have you been doing this stuff?  I ask because, unless you've been doing endurance sports for a few years, you're probably not going to do yourself any good by adding in speed work at this point.  You will naturally get faster as you swim/bike/run more.  I personally would not use weight lifting to add speed.  For anything less than an 800 meter run, you're not going to gain any speed benefit from lifting weights.  There might be other reasons to lift weights, such as weight control, but speed is NOT one of them.

I would say you're probably better off by increasing volume, and frequency, over a gradual period of time, adding in some speed work when you're about 10 weeks out from a given race.

Winter will be the perfect time to really work on your base endurance, which will allow you to do speed workouts later in the season. 

Ditto the above.  Based on your logs, it looks like your best bet is to build a more solid base in all 3 sports.  Looks like you might have a moderate running base, but not much of one in swimming or biking.  Building more endurance should be your primary focus.  That will naturally allow you to hold a higher "speed" in races.

There are things you can work on now to make speed feel more "natural".  For example, in running do some strides (short bursts, about 30sec, of 5k-type speed focusing on good form and quick leg turnover) once a week and work some tempo pace into your long run.  But the key is really building more endurance.

Don't worry, you're not alone in that sentiment.

Unfortunately, and I'm sure you probably recognize this on some level, there are no short cuts.  It takes time, dedication, and consistency.

We all want to get faster.

I think riding more really is the easiest thing to improve on...see folks that double their riding from one year to the next and they are faster.

Running/swimming more helps but I don't see such a direct correlation. Swimming is very technique oriented.

I'm 45 and when I do "speed" running workouts I do what others suggest strides of run faster to next mail box, do hill repeats, stuff like that. Warming up is key and it is uncomfortable. My first year I had a coach the workout I would skip  was running intervals as they were unpleasant. It took me awhile to figure out I was doing that..as I always had an excuse, then I renamed them to my favorite workout ...sounds maybe stupid but a lot of training/racing is mental at least for me. It took maybe 3 months until I figured out how to do them well and looked forward to the challenge as it did get me faster...that was last year in my 3rd season...2nd with a coach. This year no speed work for me running as I've been recovering from different injuries most of the year.

HOWEVER ........

There are some things to keep in mind.

Depending on your physiology/genetics you may be pre-disposed to have more fast twitch muscles than others might have (whether they are "A" or "B" type).

By the sheer fact that you are doing an endurance activity you will be training your type "A" fast twitch to take on the characteristics of a slow twitch fiber and the more that you can "train" to do that the better endurance athlete you will be.  That is done through volume and repetition at an aerobic rate.

If you don't do any sort of speedwork to really stimulate your Type "B" fibers than you run the risk of losing their effectiveness.  What does that mean to you??  Well ...... you will not be able to charge up that hill on a run or sprint for that next stop sign on the bike as easily or effectively.  The best example of this is that marathon runner who can run 6:30's all day long, but gets whooped at the finish because he/she has no sprint.

The age question is not that much of a factor other than that to maintain the speeds that come easily as a younger person you will just have to work a little harder at it.  Meaning you'll have to train a little more (and smarter) to effectively use the different muscle fibers than a younger person.

Be patient, unfortunately to develop a solid base for endurance sport it will take time (read years) Indeed doing intervals at VO2 will improve your speed (VO2Max or peak) but within weeks you will stop seeing noticeable benefits and in order to improve furthermore you’ll have to push harder and harder until you basically reach a breaking point.