hi lazerpixie,

It is really hard for me to comment on what you can and can't do. There simply isn't enough information for me to tell and I haven't seen you personally.

What do you think you can manage? Do you think it will cope or get worse if you increase your mileage?

Do you normally run 21km in VFF or have you just started?

How heavy are you?

How is your foot alignment?

Are you strong enough to run barefoot?

How elite or fit are you? Asking a proper, long time, properly coached runner to push thru the main might be ok in some cases but for the average punter, i think it isn't very wise...

Lastly, it just doesn't sound like the whole picture points to Wikipedia reference-linkcuboid syndrome...this is how they seem to present to me in my clinic (and rarely overall really)...
1. Twisting of the foot while loading the leg and foot
2. Can't put weight onto the outside of the foot - usually happy to walk on the heels, not toes
3. Manipulation causes INSTANT relief of pain and restores ability to walk again with only minor soreness compared to sharp pain
4. taping allows normal activities to occur
5. no pain anywhere else in the leg or foot - just over the prominent cuboid.

Now i realise you can get cuboid sprains without the subluxations etc but it is essentially the same thing. If it isn't like the description above, to me, it isn't really cuboid syndrome - i would go and check you from your neck to your thorax, spine, pelvis, hips, knees, ankles and feet to work out why you have the pain...

...but that's just me

Best to trust your gut...what do you think?