I've been doing some reading in running forums lately and I've picked up on a few things that these particular individuals seem to be doing that is really harming their progress.
People who are heavily into running take on a very different attitude oftentimes than those who are heavily into weight lifting. However, there are also individuals that do try and combine the best of both worlds, doing some running and then also participating in strength training programs as well.
While obviously if you are trying to be very competitive with weight lifting or are really looking to pack on the muscle mass, performing too much running is certainly going to hamper your progress. You should still include some form of cardio however it should be in a mode other than running or strictly kept to high intensity sprint running work.
For most however, you can find a decent middle ground and the running you will do will not harm your weightlifting all that much, if anything it may benefit it. This is also assuming of course that you are following smart training protocols, which unfortunately many runners are not.
Even for those who are heavy duty runners, they should still abide by many of these rules as they will apply to them as well.
1 / Eating Too Little Protein
This is probably the number one error I see. Some of the runners on these forums are eating as little as 50 grams of protein a day. That's barely enough to support a small child let alone someone who is exercising strenuously.
The RDA is set for 0.8 grams of protein per kilogram of body weight however this assumes two things. First that the person is taking in enough calories and second that they are sedentary. Often times neither of these two conditions are being met (see mistake #2).
What runners need to realize is that often they will require even more protein than a strength training athlete would. The reason for this is because when they go on their distance runs, if they are short on calories their body will start using its dietary intake of protein for fuel. This than means that it is not available for tissue growth and repair, leaving the individual protein deficient.
Generally to avoid muscle loss, they should aim for at least one gram per pound of body weight, or more preferably, 1.5 g/lb. This will help to ensure that there is no nitrogen lost from the body (nitrogen balance means no muscle tissue is being lost).
If you are eating enough calories to support your activity than you can likely move this down to one gram per pound however it would be smart not to move below this.
2 / Not Eating Enough Calories
Generally runners come in two camps. The first is the deprivers and the second is the: "I run therefore I can eat whatever I want" group. Lets discuss the deprivers.
This is the second biggest mistake many runners make is simply not eating enough calories to begin with. Even if they are trying to lose weight they often take their calories so far below where they should be that their metabolisms start to slow down, their weight loss results will come to a halt and of course performance will begin to suffer.
Those who are running 3+ times a week should never be eating below 11 times body weight. For example, if you are a 140 pound runner, always consume at least 1540 calories (11X140) if you plan to continue running. This will help to ensure you are getting enough calories without seeing a huge metabolic slowdown.
3 / Eating Too Many Calories
Likewise, as mentioned there are the runners who feel that since they run, this gives them the free for all when it comes to the table.
These individuals are drastically overestimating how much they are burning while they run and underestimating how much food they are actually eating. To make the problems worse, they more often than not choose to load up on carbohydrate rich foods such as pasta, bagels, bread, granola bars, ice cream, etc. This is going to add unwanted weight to their frame and send their energy levels on a rollercoaster.
It is very important that these particular people spend some time learning how many calories are in particular foods and getting a better grasp on just how much they are burning during their workouts. They would likely be far better off consuming their extra calories as protein and fat making their diets more balanced (while still reducing TOTAL calorie intake).
Conclusion
So if you are a runner, either a serious runner or a weight lifter who has a side passion for the sport, make sure you are not making these errors. They are all too costly and will not only jeopardize your progress but will also make maintaining a healthy weight much harder.