An article by Jeff Gaudette over at Runners Connect actually quantifies how long it is until you feel the benefits of a running workout. It is extremely interesting and not just for runners. Anyone involved in endurance sports should find it provocative. Jeff Gaudette is very knowledgeable and I find most of his material very credible so I feel the information is reliable.
In general, I was amazed at how long things take. For example, according to the article, it takes 4-6 weeks for you to benefit from a hard long run. That's much longer than I would have expected. However, it only takes 1-3 days for a speed development workout. Speed development refers to sprint workout...drills...plyometrics...things that develop pure speed. The time frame for this is short because the improvement is mostly neurological...and that sort of development happens quickly. That's why your apparent strength goes up so quickly when you start lifting weights. All the immediate gains are due to neural improvements...as opposed to building muscle.
This explains many runners' fascination with and emphasis on speed work. Speed work brings quicker results. The longer less intense stuff takes much longer. You can also see why scientific studies with short time frames will tend to result in more favorable results from high intensity training.
Any way you look at it...training is a process...and it seems to take some time. I guess we should keep things fun along the way.
"The two most powerful warriors are patience and time."
Leo Nikolaevich Tolstoy