There is no getting around the fact that long runs are crucial to a successful marathon training program.
1. Long runs do not have to be fast. They are about building "resistance to fatigue", not speed.
2. You don't just start at 22 miles so there must be a build-up phase. You would be safe by adding 1-2 miles per week to your long run, up to an absolute minimum of 15 miles. Depending on your speed, I would recommend stopping most long runs at 2 hours, and never exceeding 3, even if your projected time is 4 hours or slower.
3. Surface. I believe that if you do most of your long runs off-road you recover faster, however you still need some time on roads to simulate the race. Ideally this would be on firm grass or trails that allow you to run at a respectable pace. I've found that many trails, especially those that are hilly, rocky, or at altitude, are run at a pace that is just too slow. If you can only cover 10 miles in 2 hours and hope to break 3 on race day, you might be better off back on the roads.
4. Timing. Outside of the serious competitors, most marathoners don't have the time or desire to be doing long runs year round, so they must time their progression according to the race date. Starting at 10 miles, you should be able to reach 18 miles in 8 weeks.
5. How long? I favor 18 miles as the upper limit for long runs, because the recovery is so much faster than a 20-22 miler and I am not convinced there is any significant added physiological benefit from the longer distance- in fact unless you are an extremely efficient, light weight runner, you may be doing more harm than good.
6. How many? Here is where my opinion varies from most marathon progression programs. I like to build up to 18 miles, and then maintain that distance for 4 consecutive weeks. Ideally, your first 18 miler comes 6 weeks out from your race, and your last one 2 weeks out. That's a little late, but like I said you recover faster from 18 than you do 20, especially since you've already put 3 under your belt. Here is my 16 week progression for weekly long runs beginning with 8 miles:
Week 16: 8 miles
Week 15: 9 miles
Week 14: 10 miles
Week 13: 11 miles
Week 12: 12 miles
Week 11: 13 miles
Week 10: 14 miles
Week 9: 15 miles
Week 8: 16 miles
Week 7: 18 miles
Week 6: 18 miles
Week 5: 18 miles
Week 4: 18 miles
Week 3: 12-13 miles (race simulation)
Week 2: 12 miles
Week 1: 10 miles
I also like 18 miles because the 9 minute per mile runners can still complete the distance in under the 3 hour limit (2 hr 42 min at 9 min pace).
Note the race simulation with 3 weeks remaining. Here is where you test your capability for maintaining marathon pace or faster for about half the distance. This won't prepare you for what's in store the last 6 miles, but can be a big confidence booster none the less. If you can schedule a half marathon race here, do it.
7. The long run is more than just about putting in the distance. Here is where you experiment with shoes, clothing, hydration and what form of calories work best for you (gels, liquid, or something else). Once you get beyond 15 miles, I would break out the shoes I plan to wear on race day for a test run or two, just to make sure nothing unexpected happens after 2 hours in them.
8. WEIGH YOURSELF BEFORE AND IMMEDIATELY AFTER ALL LONG RUNS! This is overlooked by the majority of marathoners, but avoiding dehydration is absolutely essential in preventing "the wall." You must try to drink enough to minimize large drops in body weight during long runs (4% or more). If you weigh 150, an acceptable loss would be 3 pounds (2%). Lose more than 6 pounds (4%) by 20 miles and you may have a long 6 miles ahead.
9. BE FLEXIBLE! There are many variations to long run build-ups. For example, Jeff Galloway recommends building up to distances of 26 miles and even longer, allowing a full 2 weeks in between. Hal Higdon uses 20 miles, as do most other programs (the most significant difference is he only suggests 2 runs 18 miles or longer in the last 6 weeks, and I like doing 4).
10. BUY A GPS OR MEASURE! Back in the day before GPSs, I used to estimate miles run based on time and how fast I thought I was running. More often than not I was off by 1 or maybe 2 miles on long runs, usually going shorter than I thought. Unless you measure or have a GPS, your 18 miler may actually be 16!
(c) Dave Elger 2008 All rights reserved