Unfortunately there's a big prognostic difference between stress fracture and ligament strain. The former is less common but more serious (could be six weeks). The latter would respond to rest and anti-inflammatories then graded rehab with stretches and orthotics if indicated. I saw my podiatrist 10 days ago and am back in training with some customised inserts. If things dont settle rapidly and a stress fracture is a possibility an XR is required (although ultrasound over the site from a physio is usually painful +++). As for other training -yes - keep cardiovascular fitness with non weight bearing exercise and plenty of stretches.
FBF - saw podiatrist Saturday and as expected biomechanical problems at root of things. Basically old football injury to L ankle causing inversion, and 1st ray on both feet out of alignment with 2nd-5th metatarsals (and I though I had quite nice feet actually !!) Been given some customised orthotics and stretches to do. Ran 5 miles that afternoon and 10 miles yesterday with very little reaction so hopeful all will be well. Funnily enough this month's RW has an article for the "older runner" and at 42 I realise now that they mean me !! Been following a 4 run/week schedule but now realise must look at cross-training to save the old legs a bit. Hope you are progressing well too.
Foot improving with rest (6 days without a run now) - going to test things gently over the weekend and see the podiatrist next week if I get a reaction. Coke can sounds good - it's the basics we all forget - I'm always fitting a run in to a tight schedule and tend to neglect the warm down and stretches. Ice to an injury in the acute phase obviously makes sense but I've not done it - and I'm supposed to be the medic!!
Marion - do go to see a podiatrist - there is a thread under FLM 2003 called plantar fasciitis that also touches upon the kind of symptoms you describe. The main point is that orthotics need to be personalised to the individual and no "one size fits all" exists. Once the nastier things have been ruled out a biomechanical assessment may define a gait problem that is specific to you and a personalised orthotic can be designed. They are expensive but last for ages and really do work.
As for me - it will be my first ever London and my first marathon since Bradford in 1983. The foot is still a bit sore after 5 days rest but I'm going to test it out over the weekend and see how it goes. Thanks for the interest.