There is a well known rule of thumb that an interference effect exists between strength and endurance training: If you are trying to get bigger muscles by strength training, adding endurance training into the mix will slow the process down. If you are trying to improve your endurance performance, throwing strength training can interfere with your efforts. There is some science to back this up, but viewing it as all or nothing is oversimplified. Recent studies addressing this were covered in Alex Hutchinson’s Sweat Science post for Outside Magazine.
Female endurance athletes (they were actually duathletes who compete in combined cycling and running events) added twice a week heavy strength training (leg work) to their programs . They gained as much strength in 11 weeks as untrained women. So the endurance regimen of the duathletes did not interfere with their strength gains. The exception was in high power movements like maximum jump height, which improved more in the “strength only group” than the “endurance plus strength group”. Unfortunately the authors did not mention the absolute performance on these tests of the two groups, it is likely the duathletes were better at these moves to begin with.
The other crucial finding (by the same authors, published separately) was that the endurance plus strength trained athletes improved their fatigue resistance in running and cycling tests compared to a group of athletes who trained endurance only . So the addition of strength training was beneficial, it did not interfere with endurance. It is also widely thought that the addition of strength training to an endurance program helps with injury prevention.
My take is to mix a modest amount of leg strength training with my endurance training. I do a larger amount of upper body strength training because I do less endurance training of the upper body. This is a good mix which helps me stay injury free (as long as I don’t have accidents!) and prevents muscle loss with age.
I think the jury may still be out for those who are primarily interested in strength training, as to the effect of adding endurance training to the mix. Alex focussed on that in a previous Sweat Science article here. He showed evidence that the impairment in strength gains is not much of a problem unless you are doing endurance training four or more times a week. Even then, the problem can be mitigated by making sure you eat enough to offset the calories burned doing endurance exercise. You should start each strength training session with your fuel tanks topped off, and also make sure you are getting adequate protein.