Recommendations for a new strength training plan

First things first, 15-20 minutes for lifting isn't much time at all, I like to have my athletes lifting for around 45minutes per session. For strength we keep rest periods to round 2 minutes, enough time to repletion an adequate % of ATP whilst keeping the intensity of the sessions up. But if you're only lifting for 15 minutes yours only going to be able to perform 7 sets total, which really isn't going to be enough for maintenance, I'd suggest using two lunch breaks a week purely for lifting. Jung (2003) found that trained distance runners have shown improvements of up to 8% in running economy following a period of resistance training, and that resistance training didn't compromise VO₂max. So taking time away from your cardio and adding it to resistance training might be a good thing.

As for reps and weight, it's strength your want, "toned" is just low body fat you can't tone muscle only increase mass and reduce body fat, so you're going to want to keep the %RM high and the reps low, I'm talking 80% of max and 1-3 reps, with longer rest periods of 2 minutes, with 3-5 sets per exercise. 

As for the exercises, squats, RDLs, and Deadlifts are going to be the staple for lower body, and upper body it's going to pull-ups, rows and shoulder press. Nothing fancy but they'll work the key muscle groups used in triathlons, Bench press doesn't added to performance, at no times during an event our we pushing, in swimming we're pulling hence the inclusion of a number of pulling exercises.

I suggest picks 3 lifts per session, one upper, one lower, and one complimentary. Keep the same exercises for a couple of weeks and then changes them for something else. Strength training isn't fancy and the exercise are old, but they're still around for a reason.

Thanks!