marathon and knee problems

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I've been in training for a long while now and have been following a schedule for a marathon. Two
weeks ago I did a long run (17½ miles) and felt pretty good afterwards apart from being knackered
but last sunday I was doing my last long run before the actual race and about half way through my
knee suddenly started hurting and I had to cut short the run. It's still hurting now on the outside
and on the bit at the back of the leg although the pain is disappearing as the day progresses. When
I stopped running yesterday I actually found it easier to jog gently rather than walk so I just
jogged home slowly.

I'm now a little worried that I've done some damage to my knee and wont be able to take part in the
run. Until this year I've only ever ran short distances (less than 10 miles) with any frequency so
long distances are not what my body's used to.

It's been my ambition to this marathon for years and I'm doing it this year to co-incide with me
being 40. I also feel that if I don't do it this year then I wont do it ever. However nothing's
worth messing my legs up for and I'm wondering if anyone can offer some general advice/experience of
what they would do/have done in this situation.

What I feel I could do is forget doing any long runs now and just go to the gym and use the
treadmill (on the grounds that the treadmill is gentler to the body than running on roads) for
shortish runs until the marathon week. scott