Before the strength trainers and fitness industry fanatics fire up the torches or gather stones to throw, I have nothing against lifting weights. This is a matter of specificity. I've had much more success with time-limited athletes when I've focused the time they have on workouts that address the unique demands, and ranges of motion, specific to their sports.
If you're a runner, cross country skier, swimmer, cyclist, or paddler, technique and economy of motion play a huge role in your performance. When you split your limited time between your sport and lifting weights, it's very difficult to make significant improvement in either area. You might make some small strength gains, but the hours you spent in the gym was time robbed from your endurance training. If you're a cyclist and training 10-12 hours or more each week, you may have enough time to incorporate weight lifting. For runners, this threshold occurs at about 30 miles a week. Below that, you'll benefit more by focusing your time on sport-specific workouts.
You can incorporate strength work into most, if not all endurance activities. Resistance training is an important part of maintaining muscle mass, stabilizing joints, and preserving good posture, all of which reduce the chance you'll injure yourself picking up a bag of dog food. Look for ways to increase the load or resistance you're acting against. For cyclists, this can mean 10 minutes of climbing in a big gear with low cadence. For runners it can mean bounding up short, steep hills. Paddlers can gain sport-specific strength through repeated, forceful accelerations. You should also take 10-15 minutes at the end of your endurance workout to incorporate abdominal work and low-back exercises, and learn some basic plyometric exercises you can do with medicine balls and other equipment you can keep around the house.