Over the years, i've trained athletes of all backgrounds for countless different sports and personal goals. A common question i've gotten from my more endurance-focused clients is about how strength training will affect their long-distance work, or if they can do well at both.
The answer is: Yes.
This question also comes up a lot with military clients who need to improve or maintain running performance for physical fitness tests. Marines, for example, run a 3 mile test as part of their PFT, with a maximum score of 18:00 and a minimum of 27:40. Invariably, strength training, when done correctly, vastly improves run times in these athletes.
Even if you aren't part of an organization that requires you to be able to run fast, you might still want to improve or maintain your endurance ability for any variety of reasons. The question is: how will adding a serious strength progression to your training affect this?
The best analogy I ever heard was this:
Does putting a bigger engine in a car slow it down, just because the engine is heavier?
No, of course not.
If you don't believe me, check out the myriad of studies that have been done on strength training for endurance athletes, like this one.
As long as we maintain your cardiovascular ability through intelligent endurance and short-distance interval training, getting stronger or bigger, especially in the legs, will only make you a better runner.
Will you require slightly more energy from food to maintain your new muscle and make it work for you? Yes. The keyword is slightly. Once you've built the strength and size, it's much easier to maintain than it was to get there in the first place.
Another consideration is periodization of training. There will be times during your training that strength work will need to be prioritized over your endurance work. This is NECESSARY to achieve the end result of fast/endurance + strong. Strength, as in, your ability to move heavy loads, is a much slower adaptation - but the good news is that means it's also going to stick around longer.
Endurance, on the other hand, comes quickly but also dissipates quickly. Taking time off of running will be more detrimental to your endurance than taking time off of lifting will be to your strength. This is fine because if you're an endurance athlete who needs to put the 10km runs on hold for a couple of months while you get your squat up, you'll be able to get back to your previous performance times in relatively short order (and, if things go right, surpass them.) Usually, my clients don't experience any decrease in endurance performance anyways. It all depends on the athlete's ability to recover, how advanced they are in their running training, etc.
So, even if (and that's a big if) your endurance takes a slight dip while we're prioritizing strength, don't worry - a few weeks of refocusing will have you back where you were, only FASTER now due to your increased leg strength.
If every step you take when you run is X percent of your "max effort" leg strength to step, and we increase your leg strength, each step becomes a smaller percentage of your max effort. Simple.
Now, you don't need to completely stop all endurance training while you're getting stronger. A more likely scenario is you'll just need to cut back on overall weekly mileage to facilitate strength adaptations. I do this in a pre-planned way with my athletes. Oftentimes, they don't experience any decrease in endurance performance at all during this time.
If you're an endurance athlete or wish to maintain your ability to go long, you should be prioritizing your strength training as a means to that end!