There is scientific evidence to support the opposite... Essentially Hans Selye put forth a basic theory around 1925 that governs our understanding of stress and adaptation called General Adaptation Syndrome. This theory has provided the foundations of both vaccinations and all exercise theory. In essence, when your body is put under some form of stress, it needs to adapt to that stress to better handle it the next time. This is why injecting a weaker poison in your system can improve your immune response to a similar strong poison. It's also why we get stronger when we lift heavy objects.
We've refined this basic underlying theory with a better understanding of our bodies, which is nicely summed up in the book Practical Programming for Strength Training by Dr. Kilgore and Mark Rippetoe. The important bits are:
- Exercise is specific. Working your arms doesn't make your thighs get stronger.
- There are several metabolic pathways to get energy to your muscles. Not all exercise causes those energy systems to adapt the same way.
- There is a difference between strength and endurance.
Now, as Rippetoe is fond of pointing out, more strength will only help you in all areas. However, there is a marked difference in the type of strength necessary to run for 4 hours straight and the much shorter intense exertion necessary in weight lifting. In fact, most endurance efforts require aerobic conditioning. This is in sharp contrast to the anaerobic pathways needed for sprinting (in contrast to your marathon run).
Because exercise is specific, and you need to optimize your aerobic metabolic pathways, you need to select exercises that are compatible with the type of training you need for long distance running. Compatible endurance oriented exercise includes:
- Running (long distance)
- Cycling (long distance)
- Swimming (long distance)
Look familiar? That's right, the three most common compatible exercises are what makes up the triathlon. Weightlifting would actually work against you because it is an anaerobic activity. Cycling and swimming can help you in that they can provide more resistance, causing you to work harder. That in turn will help your legs get stronger while still using the same metabolic pathways, causing you to put forth less effort with each stride. The same will be the case if you keep increasing the incline on your treadmill.