Building a long run

Hi everyone I have an autumn marathon (Abingdon) and am up to about 14 miles on my long run. What I want is to build up my long run and be able to safely maintain it. If I add a mile each week (with a back down week after 3 weeks), I'll be up to 20 miles by summer. In theory, that would mean I would then spend a few months doing a 20 every Sunday, which seems a bit much.

So... my question is this; what is the best thing to do? Stay where I am for a few months before building up, or alternate a longer run with a shorter one (12 then 16, 12 then 17, etc)? Or something else? What do people who run multiple marathons do to maintain a long run?

Appreciate your thoughts, Nick.

If you get into the mindset that everything between now and Abingdon is a big long gradual marathon build-up you'll probably get stale at some stage.  Having said that, it's probably a good idea to keep building up but do it in steps with a few cut-backs.  I'd add a mile or two every other week till you're up to about 18, but have a cut-back every 3 to 4 weeks to 12-14 or so.  Then depending on how many 20+ LRs you want to do - I don't think there's a right or wrong answer and it depends to some extent on your endurance background and fitness level - count back from race day to work out how soon you need to be hitting 20 miles.  Once you're up to that sort of distance, you don't want to be doing 20 mile runs week-in, week-out, so definitely schedule a mid-length run every 3 weeks or so.

Another way to break things up is with warm-up races.  Get some 5k or 10k races in over the summer, then maybe a HM about 4 - 6 weeks out from the marathon.  Build a mini-taper plan around the HM and maybe one target 10k a few weeks before, and this will take care of some of the cut-backs in the plan.

Sample, off-the-top-of-my-head possible Sunday mileage plan:
14, 15, 15, 12, 15, 16, 12, 17, 18, 14, 18, 20, 10k race, 18, 20, 12, 20, 20, HM... you get the idea. 

Thanks for your input Phil.

From what I've read, it's a good idea to have an 'easy' week every third week. I've been building up since the start of the year and my long runs are now up to 20 miles. I've got my first big event of the year at the end of May and my last one at the end of September.

So for the next few months I'm planning on my long runs being roughly 20-22 one week, 18 the next week, then 13-15 the third week. (And of course the tapering weeks for each race.) On the days round about the shorter long run, I'm going to work more on speed and on the days round about the 20-22 miler, I'm going to just concentrate on getting the miles in and not worry about how fast I'm going. 

And of course, because you're going to be up to the 20 mile distance months before your marathon, you won't need to worry so much about sticking to a strict training plan. You'll be able to run according to feel and stick in a few extra miles on weeks when you're feeling great and cut back a bit on weeks when you're feeling tired.