Obviously, longer runs require longer recovery. Besides using something like gels and drinking water when running, is there anything else I can do to help my body from being depleated as much during long runs so I can recover faster afterwards?
When I was doing my distance training I found a litre of chocolate milk was a great cheap recovery drink. Lots of carbs, protein, tasty, and available everywhere.
My friends find it amusing that every time an article is published suggesting this, they send me links, which I alas do not store.
But soon (15-30 minutes) after your long run is finished, have some carbs and protein. Chocolate Milk is just an easy thing to handle at that point.
In addition to hydrating, replenishing energy stores, etc., I have found that my recovery time is improved when I don't overextend myself. Two things I've found that help to keep my recovery time in check are: not attempting to run a distance that is further than I'm able to handle, and not running at a pace that is too fast for my goals.
Generally speaking, it is recommended that long runs not exceed 25-35% of your weekly running volume. This distance is usually adequate for gaining all of the benefits of the long run, while not requiring such a lengthy recovery that it interferes with training. Depending on your goals, there are times where it makes sense to exceed this, but the range is a good rule of thumb.
Regarding pace, again, this depends on your goals, but for a typical long run, it is recommended that the pace be 1-2 minutes per mile slower than your goal marathon pace. Another guideline would be to keep the pace "conversational." Basically, the faster/harder you go during your long run, the more time you'll need to allow for recovery.
One last thing I've found to help is to go for a short, easy-paced run the day after a long run, instead of taking a day off.
As always, pace and distance should adjusted based on your ability to complete a workout on any given day. Listen to your body--have you been getting adequate rest and nutrition? Consider the elements--is it warmer or more humid than what you're used to?