My best suggestion is to talk with someone who really knows how to structure a training program for your particular goals, position, and body makeup. I'm assuming you're asking mostly about baseball since you're here. One thing you absolutely won't succeed at by running cross country and playing alot of basketball (another high-endurance running sport) is put on much muscle mass.
While running is certainly a part of baseball ... short distance sprint speed is what's important, not running endurance, so distance running isn't really helping your baseball game. Sprint work, great ... but running 15 miles will just force your body to convert to lean muscle mass and reduced body weight to maximize the efficiency with which your body carries that weight over long distances, neither of which helps you hit with more power or throw harder. Just the opposite.
My son played basketball is Frosh & Soph years and even after working hard in the weight room and paying attention to nutrition, he just couldn't put on much weight because he just ran it off. After that he really focused on weight training that was specifically designed for his body makeup and muscle groups important to pitching and hitting. Legs, back, core torso, forearms. Always careful not to get bulky, but strong. No power lifting stuff. This was complemented with yoga, balance work (dumbell workouts standing on a circular platform placed on top of a cylinder ... worked fine motor control of multiple muscle groups), stretching, core strength training on the off days between. This helped with flexibility, balance, and keeping the gained muscle mass longer and more resilient.
One thing he absolutely stopped doing is running long distances. He gave up basketball. He still ran enough to have good cardiovascular and did wind sprints, but he decided to set a goal to focus on developing his body makeup for baseball ... and specifically for pitching and power hitting.
Just look at the body style of your typical marathon runner and your typical pitcher or top hitter. Completely different body shapes.
If you're having trouble with motivation in the weight room, try getting into a regular program with a buddy that has similar goals so you can encourage and push each other, or get into a conditioning night class for ball players. If spending $$$ isn't an issue you can work with a trainer to help push you ... but maybe just for a short time to help get you set up in a program.
In the end, it all comes down to what your goals are. Playing multiple sports is a wonderful thing, so you just have to decide what's most important for you.
