[jules]

Running is way more exhausting than weight lifting. If you're not exhausted, run faster and longer. You can't do the same with weight lifting. You can get your arms tired, but your arms will give up long before you run out of energy.

[kareemm]

If you're lifting weights with your arms, you're probably doing it wrong.

Try the olympics lifts: power cleans and snatches and squat cleans and snatches. Or their components: deadlifts, front squats, presses. Or back squats and thrusters. Do a bunch of those in rapid succession and your whole body will be much more tired than doing a 5k run, guaranteed.

[jules]

This is not my experience. After a couple you won't be able to lift anymore, but you won't really be exhausted either. At least I can't lift any more just because my muscles fail. I don't regularly lift weights though, so perhaps if you're more experienced you can. That said, you don't need experience to get exhausted running (but the tendency of many people is to stop long before that point).

[achompas]

You're probably lifting too much weight, or leaving the gym too early. I know this isn't helpful, but you should be exhausted once you leave the gym.

[jules]

How much weight should you lift? How long should you spend in the gym?

[achompas]

It's more complicated than a simple formula. If we're talking about olympic lifts (squats, deadlifts, clean & press), start with the bar and increase 5 lbs every day until you start hitting a limit.

Check out Mark Rippetoe's "Starting Strength" [1] for more info. Great program for anyone new to weightlifting.

[1] http://startingstrength.com/

[patrickk]

Also, running on a track or anywhere that is not a treadmill is much more exhausting than using a treadmill. The treadmill does some of the work for you. It also makes you more prone to injuring yourself, as happened to me. In short, the are much better places to run than on a treadmill.