Knees - Iliotibial band syndrome

Hi,

I'm training (for the first time) for the London Marathon - training has been going really well but suddenly have pain on the outside of my knee - looking online think it could be the iliotibial band.

I am only a few weeks away so obviously want to keep up fitness, but know I should probably rest. What is better - to rest entirely, or to do some cross training (the cross trainer / swimming / yoga / weights) to keep my legs happy?

Any advice would be hugely appreciated.

Thanks

harriet