What you should have as part of an ulra-light endurance-centered medical kit:
Ibuprofen: for low-level inflammation control during long-term endurance bouts that are hard on joints (running, hiking, strenuous biking). We call it "Vitamin-I" because we take them like vitamins during races and long training days. It's really important to keep swelling down during long exercise, because inflammation changes the biomechanics of your knees, ankles and hips. Your body is trying to heal itself, but you're not done with it yet! This process makes injury far more likely. So, take plenty of ibuprofen for long high-intensity exercise.
Naproxen Sodium (Aleve): this stuff is ibuprofen's bigger brother. Take a few for when you do injure yourself, it'll help you make it home without further injury.
Hammer Endurolytes: these electrolyte tabs help to restore normal electrolyte balance. Take these for when you or a friend feel like cramping is coming on. They also help you stay hydrated in the heat (but you need water for that too!).
Duct Tape: Gotta have a few feet of this stuff. This helps put stuff back together and also prevents blisters. Most recently I used some of this to tape my busted bicycle frame back together.
Super Glue: Take a "single-use" tube of this stuff, I've used it to keep stuff from falling apart on my bike. You can also use it for drastic wound closure if you know what you're doing.
Putting it all together: get a tube of Nuun or U, use it up, and take the empty tube and fill it with the pills & super glue. Wrap the outside of the tube with your duct tape to mark it as a medical kit and store your tape. Bring it with you wherever you go!