Not sure what the heck "plyometricals" are but PLYOMETRICS (along with HEAVY resistance training...ie: 1-3 sets 6-12 reps) have been PROVEN to either a) improve running economy and/or b) improve running performance in the 3k and/or 5k.
Really there are few studies that look at performance but many that look at running economy, vo2max, etc. In these studies Vo2max remains unchanged while improvements come in running economy.
I think performing plyometrics without ever doing any strength training is like doing interval training without ever running easy mileage.
So, I would suggest two strength training workouts during the off season: one day of weight training and one day of plyometric training. Once your running training becomes more intense then you would combine the two into one workout, but reducing the volume of both. Either way, you should not be in the weight room or doing plyos for more than 30 minutes.
Off Season:
Tuesday: Weight Training
Squat
Bench Press
Dumbbell Row
RDL
Shoulder Press
Lat Pulldown
*Perform as a circuit, 2-3 sets, start with a weight you can lift for 12 reps (ie: reach failure or near failure at 12 reps). Over the course of 4-6 weeks increase the weight and drop the reps...10...8...6...4. First set is always warmup, so undershoot the first set.
Friday: Plyometric/bodyweight training
Double Leg Hops for distance (30-60s)
Box Jumps (30-60s)
Pushups (AMRAP...as many reps as possible)
Single Leg Hops for distance (30-60s)
Split Jump on a box (30-60s)
Interted Row on a smith machine or another waist high bar (AMRAP)
*Again perform as a circuit
In-Season:
1. Box Jump to Depth Jump onto another Box Jump then Depth Jump into a Split Jump on the ground (both Legs)
*Set up two boxes. Jump onto one. Off to the ground. Onto the second. Off to the ground. Then Split Jump land right. Then Split Jump land left. This is to be done carrying the momentum from one jump to the other
2. Jumps Over a Box
3. Split Squat or Step Up w/hip flexion
4. Dumbbell Alternating Incline Bench
5. RDL
6. Chest Supported Row
*1-2 sets, 4-6 reps
Alan, BS, CSCS